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+.\" For tolerably obvious reason, this needs to be processed through PIC.
+.\" It also needs to be processed through TBL and EQN.  Use "groff -p -e -t".
+.\" There is no hope that this will ever look right under nroff.
+.\"
+.\" Comments beginning with %% are cut lines so portions of this
+ \" document can be automatically extracted.  %%TUTORIAL%% begins the
+.\" tutorial part; %%REFERENCE%% the reference part.  %%POSTLUDE%% the
+ \" bibliography and end matter after the reference part.  
+.\"
+.\" This document was written for free use and redistribution by
+.\" Eric S. Raymond <esr@thyrsus.com> in August 1995.
+.\"
+.\"	$Id: pic.ms,v 1.1 2000/02/06 09:39:20 wlemb Exp $	
+.\"
+.\" Set a proper TeX
+.ie t .ds tx T\h'-.1667m'\v'.224m'E\v'-.224m'\h'-.125m'X
+.el .ds tx TeX
+.\"
+.de CE\"	Centered caption for figure.  Assumes previous .KS
+.ce 1
+Figure \\n(H1-\\$1
+.sp 1
+.KE
+..
+.\" Definitions end here
+.TL
+Making Pictures With GNU PIC
+.AU
+Eric S. Raymond
+.AI
+<esr@snark.thyrsus.com>
+.AB
+The \fBpic\fP language is a \fBtroff\fP extension that makes it easy
+to create and alter box-and-arrow diagrams of the kind frequently used
+in technical papers and textbooks.  This paper is both an introduction
+to and reference for \fIgpic\fP(1), the implementation distributed by
+the Free Software Foundation for use with \fIgroff\fP(1).
+.AE
+.\"%%TUTORIAL%%
+.NH 1
+Introduction to PIC
+.NH 2
+Why PIC?
+.PP
+The \fBpic\fP language provides an easy way to write procedural
+box-and-arrow diagrams to be included in \fBtroff\fP documents.  The
+language is sufficiently flexible to be quite useful for state charts,
+Petri-net diagrams, flow charts, simple circuit schematics, jumper
+layouts, and other kinds of illustration involving repetitive uses of
+simple geometric forms and splines.  Because these descriptions are
+procedural and object-based, they are both compact and easy to modify.
+.PP
+The \fIgpic\fP(1) implementation of \fBpic\fP is distributed by the
+Free Software Foundation for use with their \fIgroff\fP(1)
+implementation of \fBtroff\fP.  Because both implementations are
+widely available in source form for free, they are good bets for
+writing very portable documentation.
+.NH 2
+PIC Versions
+.PP
+The original 1984 pre-\fIditroff\fP(1) version of \fBpic\fP is long
+obsolete.  The rewritten 1991 version is still available as part of
+the Documenter's Work Bench module of System V.
+.PP
+Where differences between Documenter's Work Bench (1991) \fBpic\fP and GNU
+\fBpic\fP need to be described, original \fBpic\fP is referred to as
+"DWB pic".  Details on the history of the program are given at the end
+of this document.
+.PP
+In this document, the \fIgpic\fP(1) extensions will be marked as such.
+.NH 1
+Invoking PIC
+.PP
+Every \fBpic\fP description is a little program, which gets compiled
+by \fIpic\fP(1) into \fIgtroff\fP(1) macros.  Programs that process or
+display \fIgtroff\fP(1) output need not know or care that parts of the
+image began life as \fBpic\fP descriptions.
+.PP
+The \fIpic\fP(1) program tries to translate anything between \fB.PS\fP
+and \fB.PE\fP markers, and passes through everything else.  The normal
+definitions of \fB.PS\fP and \fB.PE\fP in the \fIms\fP macro package
+and elsewhere have also the side-effect of centering the \fBpic\fP output
+on the page.
+.PP
+Other details of the \fI[gt]roff\fP(1) interface
+.NH 2
+PIC Error Messages
+.PP
+If you make a \fBpic\fP syntax error, \fIgpic\fP(1) will issue an
+error message in the standard \fIgcc\fP(1)-like syntax.  A typical
+error message looks like this,
+.KS
+.DS
+.CW
+pic:pic.ms:<nnn>: parse error before `<token>'
+pic:pic.ms:<nnn>: giving up on this picture
+.DE
+.R
+.KE
+.LP
+where <nnn> is a line number, and <token> is a token near (usually
+just after) the error location.
+.NH 1
+Basic PIC Concepts
+.PP
+Pictures are described procedurally, as collections of objects
+connected by motions.  Normally, \fBpic\fP tries to string together
+objects left-to-right in the sequence they are described, joining them
+at visually natural points.  Here is an example illustrating the
+flow of data in \fBpic\fP processing:
+.KS
+.PS
+ellipse "document";
+arrow;
+box "\fIgpic\fP(1)"
+arrow;
+box width 1.2 "\fIgtbl\fP(1) or \fIgeqn\fP(1)" "(optional)" dashed;
+arrow;
+box "\fIgtroff\fP(1)";
+arrow;
+ellipse "PostScript"
+.PE
+.CE "1: Flow of \fBpic\fP data"
+.PP
+This was produced from the following \fBpic\fP program:
+.KS
+.DS
+.CW
+\&.PS
+\&ellipse "document";
+\&arrow;
+\&box "\\fIpic\\fP(1)"
+\&arrow;
+\&box width 1.2 "\\fIgtbl\\fP(1) or \\fIgeqn\\fP(1)" "(optional)" dashed;
+\&arrow;
+\&box "\\fIgtroff\\fP(1)";
+\&arrow;
+\&ellipse "PostScript"
+\&.PE
+.DE
+.R
+.KE
+.LP
+This little program illustrates several \fBpic\fP basics.  Firstly, we
+see how to invoke three object types; ellipses, arrows, and boxes.  We
+see how to declare text lines to go within an object (and that text
+can have font changes in it).  We see how to change the line style of
+an object from solid to dashed.  And we see that a box can be made
+wider than its default size to accommodate more text (we'll discuss
+this facility in detail in the next section).
+.PP
+We also get to see \fBpic\fP's simple syntax.  Statements are ended by
+newlines or semicolons.  String quotes are required around all text
+arguments, whether or not they contain spaces.  In general, the order
+of command arguments and modifiers like "width 1.2" or "dashed" doesn't
+matter, except that the order of text arguments is significant.
+.PP
+Here are all but one of the basic \fBpic\fP objects at their default sizes:
+.KS
+.PS
+box "box";
+move;
+line "line" "";
+move;
+arrow "arrow" "";
+move;
+circle "circle";
+move;
+ellipse "ellipse";
+move;
+arc; down; move; "arc"
+.PE
+.CE "2: Basic \fBpic\fP objects"
+.PP
+The missing simple object type is a \fIspline\fP.  There is also a way
+to collect objects into \fIblock composites\fP which allows you to
+treat the whole group as a single object (resembling a box) for many
+purposes.  We'll describe both of these later on.
+.PP
+The box, ellipse, circle, and block composite objects are \fIclosed\fR;
+lines, arrows, arcs and splines are \fIopen\fP.  This distinction
+will often be important in explaining command modifiers.
+.PP
+Figure \n[H1]-2 was produced by the following \fBpic\fP program,
+which introduces some more basic concepts:
+.KS
+.DS
+.CW
+\&.PS
+\&box "box";
+\&move;
+\&line "line" "";
+\&move;
+\&arrow "arrow" "";
+\&move;
+\&circle "circle";
+\&move;
+\&ellipse "ellipse";
+\&move;
+\&arc; down; move; "arc"
+\&.PE
+.DE
+.ft R
+.KE
+.PP
+The first thing to notice is the \fImove\fP command, which moves a
+default distance (1/2 inch) in the current movement direction.
+.PP
+Secondly, see how we can also decorate lines and arrows with text.
+The line and arrow commands each take two arguments here, specifying
+text to go above and below the object.  If you wonder why one argument
+would not do, contemplate the output of \fBarrow "ow!"\fP:
+.KS
+.PS
+arrow "ow!"
+.PE
+.CE "3: Text centered on an arrow"
+.PP
+When a command takes one text string, \fBpic\fP tries to place it at
+the object's geometric center.  As you add more strings, \fBpic\fP
+treats them as a vertical block to be centered.  The program
+.KS
+.DS
+.CW
+line "1";
+line "1" "2";
+line "1" "2" "3";
+line "1" "2" "3" "4";
+line "1" "2" "3" "4" "5";
+.DE
+.ft R
+.KE
+.LP
+for example, gives you this:
+.KS
+.sp 2
+.PS
+line "1";
+line "1" "2";
+line "1" "2" "3";
+line "1" "2" "3" "4";
+line "1" "2" "3" "4" "5";
+.PE
+.sp 2
+.CE "4: Effects of multiple text arguments"
+.PP
+The last line of Figure 3.2's program, `\fBarc; down; move;
+"arc"\fP', describing the captioned arc, introduces several new ideas.
+Firstly, we see how to change the direction in which objects are
+joined.  Had we written \fBarc; move; "arc"\fP,
+omitting \fBdown\fP the caption would have been joined to the top
+of the arc, like this:
+.KS
+.PS
+arc; move; "arc";
+.PE
+.CE "5: Result of \fBarc; move; \"arc\"\fP"
+.PP
+This is because drawing an arc changes the default direction to the
+one its exit end points at.  To reinforce this point, consider:
+.KS
+.PS
+arc cw; move; "arc";
+.PE
+.CE "6: Result of \fBarc cw; move; \"arc\"\fP"
+.PP
+All we've done differently here is specify "cw" for a clockwise arc.
+Observe how it changes the default direction to down, rather than up.
+.PP
+Another good way to see this via with the following program:
+.KS
+.DS
+.CW
+line; arc; arc cw; line
+.DE
+.ft R
+.KE
+.LP
+which yields:
+.KS
+.PS
+line; arc; arc cw; line;
+.PE
+.CE "7: Result of \fBline; arc; arc cw; line\fP"
+.LP
+Notice that we did not have to specify "up" for the second arc to be
+joined to the end of the first.
+.PP
+Finally, observe that a string, alone, is treated as text to be
+surrounded by an invisible box of a size either specified by width
+and height attributes or by the defaults \fBtextwid\fR and
+\fBtextht\fR.  Both are initially zero (because we don't know the
+default font size).
+.NH 1 
+Sizes and Spacing
+.PP
+Sizes are specified in inches.  If you don't like inches, it's
+possible to set a global style variable \fBscale\fP that changes the
+unit.  Setting \fBscale = 2.54\fP will effectively change the internal
+unit to centimeters (all other size variable valuess will be scaled
+correspondingly).
+.NH 2
+Default Sizes of Objects
+.PP
+Here are the default sizes for \fBpic\fP objects:
+.RS
+.KS
+.TS
+tab(@), linesize(2);
+lb | lb
+l | l.
+.sp 2p
+Object@Default Size
+.sp 2p
+_
+.sp 2p
+box@0.75" wide by 0.5" high
+circle@0.5" diameter
+ellipse@0.75" wide by 0.5" high
+arc@0.5" radius
+line@0.5" long
+arrow@0.5" long
+.sp 5p
+_
+.TE
+.KE
+.RE
+.PP
+The simplest way to think about these defaults is that they make the
+other basic objects fit snugly into a default-sized box.
+.NH 2
+Objects Do Not Stretch!
+.PP
+Text is rendered in the current font with normal troff line spacing.
+Boxes, circles, and ellipses do \fInot\fP automatically resize to fit
+enclosed text.  Thus, if you say \fBbox "text far too long"\fP
+you'll get this:
+.KS
+.PS
+box "this text is far too long for a default box"
+.PE
+.CE "1: Boxes do not automatically resize"
+.PP
+which is probably not the effect you want.
+.NH 2
+Resizing Boxes
+.PP
+To change the box size, you can specify a box width with the "width"
+modifier:
+.KS
+.PS
+box width 3 "this text is far too long for a default box"
+.PE
+.CE "2: Result of \fBbox width 3 \"text far too long\"\fP"
+.PP
+This modifier takes a dimension in inches.  There is also a "height"
+modifier that will change a box's height.  The \fBwidth\fP keyword may
+be abbreviated to \fBewid\fP; the \fBheight\fP keyword to
+\fBheight\fP.
+.NH 2
+Resizing Other Object Types
+.PP
+To change the size of a circle, give it a \fBrad\fP or \fBdiam\fP
+modifier; this changes the radius or diameter of the circle, according
+to the numeric argument that follows.
+.KS
+.PS
+{circle rad 0.1; move down 0.2 from last circle .s; "0.1"};
+move; circle rad 0.2 "0.2"; move; circle rad 0.3 "0.3";
+.PE
+.CE "3: Circles with increasing radii"
+.PP
+The \fBmove\fP command can also take a dimension, which just tells
+it how many inches to move in the current direction.
+.PP
+Ellipses are sized to fit in the rectangular box defined by their
+axes, and can be resized with \fBwidth\fP and \fBheight\fP like boxes.
+.PP
+You can also change the radius of curvature of an arc with \fBrad\fP
+(which specifies the radius of the circle of which the arc is a segnmment).
+Larger values yield flatter arcs.
+.KS
+.PS
+{arc rad 0.1; move down 0.3 from last arc .center; "0.1"};
+move;
+{arc rad 0.2; move down 0.4 from last arc .center; "0.2"};
+move;
+{arc rad 0.3; move down 0.5 from last arc .center; "0.3"};
+.PE
+.CE "4: \fBarc rad\fP with increasing radii"
+.PP
+Observe that because an arc is defined as a quarter circle, increasing
+the radius also increases the size of the arc's bounding box.
+.NH 2
+The `same' Keyword
+.PP
+In place of a dimension specification, you can use the keyword
+\fBsame\fR.  This gives the object the same size as the previous one
+of its type.  As an example, the program
+.KS
+.DS
+.CW
+\&.PS
+\&box; box wid 1 ht 1; box same; box
+\&.PE
+.R
+.DE
+.KE
+.LP
+gives you
+.KS
+.PS
+box; box wid 1 ht 1; box same; box
+.PE
+.CE "5: The \fBsame\fP keyword"
+
+.NH 1
+Generalized Lines and Splines
+.NH 2
+Diagonal Lines
+.PP
+It is possible to specify diagonal lines or arrows by adding multiple \fBup\fP,
+\fBdown\fP, \fBleft\fP, and \fBright\fP modifiers to the line object.
+Any of these can have a multiplier.  To understand the effects, think
+of the drawing area as being gridded with standard-sized boxes.
+.KS
+.PS
+# Draw a demonstration up left arrow with grid box overlay
+define gridarrow
+{
+	[
+		{arrow up left $1;}
+		box wid 0.5 ht 0.5 dotted with .nw at last arrow .end;
+		for i = 2 to ($1 / 0.5) do
+		{
+			box wid 0.5 ht 0.5 dotted with .sw at last box .se;
+		}
+		move down from last arrow .center;
+		[
+			if ( $1 == boxht ) then { "\fBline up left\fP" } else { sprintf("\fBarrow up left %g\fP", $1) };
+		]
+	] 
+	move right from last [] .e;
+}
+gridarrow(0.5);
+gridarrow(1);
+gridarrow(1.5);
+gridarrow(2);
+undef gridarrow
+.PE
+.CE "1: Diagonal arrows (dotted boxes show the implied 0.5-inch grid)"
+.NH 2
+Multi-Segment Line Objects
+.PP
+A "line" or "arrow" object may actually be a path consisting of any number of
+segments of varying lengths and directions.  To describe a path,
+connect several line or arrow commands with the keyword \fBthen\fP.
+.KS
+.PS
+define zigzag { $1 right 1 then down .5 left 1 then right 1 }
+zigzag(line); 
+.PE
+.CE "2: \fBline right 1 then down .5 left 1 then right 1\fP"
+.NH 2
+Spline Objects
+.PP
+If you start a path with the \fBspline\fP keyword, the path vertices
+are treated as control points for a spline curve fit.
+.KS
+.PS
+[zigzag(spline);]
+move down 0.2 from last [] .s;
+"The spline curve..."
+move right from last [] .e;
+[
+	zigzag(line dashed);
+	spline from start of last line right 1 then down .5 left 1 then right 1;
+	"1" at last spline .start + (-0.1, 0);
+	"2" at last spline .start + (1.1, 0);
+	"3" at last spline .end + (-1.1, 0);
+	"4" at last spline .end + (0.1, 0);
+]
+move down 0.2 from last [] .s;
+"...with tangents displayed"
+undef zigzag;
+.PE
+.CE "3: \fBspline right 1 then down .5 left 1 then right 1\fP"
+.PP
+You can describe many natural-looking but irregular curves this
+way. For example: 
+.KS
+.PS
+[spline right then up then left then down ->;]
+move down 0.2 from last [] .s;
+["\fBspline right then up then left then down ->;\fP"]
+move right 3 from last [] .se;
+"\fBspline left then up right then down right ->;\fP"
+move up 0.2;
+[spline left then up right then down right ->;]
+.PE
+.CE "4: Two more spline examples"
+.PP
+Note the arrow decorations.  Arrowheads can be applied naturally to
+any path-based object, line or spline.  We'll see how in the next
+section.
+.NH 1
+Decorating Objects.
+.NH 2
+Dashed Objects
+.PP
+We've already seen that the modifier \fBdashed\fP can change the line
+style of an object from solid to dashed.  GNU \fBgpic\fP permits you to
+dot or dash ellipses, circles, and arcs (and splines in \(*tx mode
+only); some versions of DWB may only permit dashing of lines and
+boxes.  It's possible to change the dash interval by specifying a
+number after the modifier.
+.PP
+.KS
+.PS
+box dashed "default";
+move;
+box dashed 0.05 "0.05";
+move;
+box dashed 0.1 "0.1";
+move;
+box dashed 0.15 "0.15";
+move;
+box dashed 0.2 "0.2";
+.PE
+.CE "1: Dashed objects"
+.NH 2
+Dotted Objects
+.PP
+Another available qualifier is \fBdotted\fP.  GNU \fBgpic\fP permits
+you to dot or dash ellipses, circles, and arcs (and splines in \(*tx
+mode only); some versions of DWB may only permit dashing of lines and
+boxes.  It too can be suffixed with a number to specify the interval
+between dots:
+.KS
+.PS
+box dotted "default";
+move;
+box dotted 0.05 "0.05";
+move;
+box dotted 0.1 "0.1";
+move;
+box dotted 0.15 "0.15";
+move;
+box dotted 0.2 "0.2";
+.PE
+.CE "2: Dotted objects"
+.NH 2
+Rounding Box Corners
+.PP
+It is also possible, in GNU \fBgpic\fP only, to modify a box so it has
+rounded corners
+.KS
+.PS
+box rad 0.05 "rad 0.05";
+move;
+box rad 0.1 "rad 0.1";
+move;
+box rad 0.15 "rad=0.15"; 
+move;
+box rad 0.2 "rad=0.2"; 
+move;
+box rad 0.25 "rad=0.25"; 
+.PE
+.CE "3: \fBbox rad\fP with increasing radius values;"
+.PP
+Radius values higher than half the minimum box dimension are silently
+truncated to that value.
+.NH 2
+Arrowheads
+.PP
+Lines and arcs can be decorated as well.  Any line or arc (and any
+spline as well) can be decorated with arrowheads by adding one or more
+as modifiers:
+.KS
+.PS
+line <- ->
+.PE
+.CE "4: Double-headed line made with  \fBline <- ->\fP"
+.PP
+In fact, the \fBarrow\fP command is just shorthand for \fBline ->\fP.  And
+there is a double-head modifier <->, so the figure above could have been made
+with \fCWline <->\fP.
+.PP
+Arrowheads have a \fBwidth\fP attribute, the distance across the rear;
+and a \fBheight\fP attribute, the length of the arrowhead along the shaft.
+.PP
+Arrowhead style is controlled by the style variable \fBarrowhead\fP.
+The DWB and GNU versions interpret it differently.  DWB defaults to
+open arrowheads and an \fBarrowhead\fP value of 2; the Kernighan
+paper says a value of 7 will make solid arrowheads.  GNU \fBgpic\fP
+defaults to solid arrowheads and an \fBarrowhead\fP value of 1; a
+value of 0 will produce open arrowheads.
+.NH 2
+Line Thickness
+.PP
+It's also possible to change the line thickness of an object (this is
+a GNU extension, DWB \fBpic\fP doesn't support it.).  
+The default thickness of the lines used to draw objects is controlled by the
+.B linethick
+variable.
+This gives the thickness of lines in points.
+A negative value means use the default thickness:
+in \(*tx output mode, this means use a thickness of 8 milliinches;
+in \(*tx output mode with the
+.B -c
+option, this means use the line thickness specified by
+.B .ps
+lines; in troff output mode, this means use a thickness proportional
+to the pointsize.  A zero value means draw the thinnest possible line
+supported by the output device.  Initially it has a value of -1.
+There is also a \fBthickness\fP attribute (which can be abbreviated to
+\fBthick\fP).  For example, \fBcircle thickness 1.5\fP would draw a
+circle using a line with a thickness of 1.5 points.  The thickness of
+lines is not affected by the value of the
+.B scale
+variable, nor by any width or height given in the
+.B .PS
+line.
+.NH 2
+Invisible Objects
+.PP
+The modifier \fBinvis\fP makes an object entirely invisible.  This
+used to be useful for positioning text in an invisible object that is
+properly joined to neighboring ones.  Newer DWB versions and GNU
+\fBpic\fP treat standalone text in exactly this way.
+.NH 2
+Filled Objects
+.PP
+It is possible to fill boxes, circles, and ellipses.  The
+modifier \fBfill[ed]\fP accomplishes this.  You can suffix it with a fill
+value; the default is given by the stule variable \fBfillval\fP.
+.PP
+DWB \fBpic\fP and \fBgpic\fP have opposite conventions for fill values
+and different defaults.  DWB \fBfillval\fP defaults to 0.3 and smaller
+values are darker; GNU \fBfillval\fP uses 0 for white and 1 for black.
+.KS
+.PS
+circle fill; move; circle fill 0.4; move; circle fill 0.2;  
+.PE
+.CE "5: \fBcircle fill; move; circle fill 0.4; move; circle fill 0.9;\fB"
+.PP
+GNU \fBgpic\fP makes some additional guarantees.  A fill value greater
+than 1 can also be used: this means fill with the shade of gray that
+is currently being used for text and lines.  Normally this will be
+black, but output devices may provide a mechanism for changing this.
+The invisible attribute does not affect the filling of objects.  Any
+text associated with a filled object will be added after the object
+has been filled, so that the text will not be obscured by the filling.
+.PP
+The closed-object modifier \fBsolid\fR is equivalent to \fBfill\fR 
+with the darkest fill value (DWB \fBpic\fR had this capability but
+mentioned it only in a reference opinion).  
+.NH 1
+More About Text Placement
+.PP
+By default, text is centered at the geometric center of the object it is
+associated with.  The modifier \fBljust\fR causes the left end to be
+at the specified point (which means that the text lies to the right of
+the specified place!), The modifier \fBrjust\fP puts the right end at
+the place.  The modifiers \fBabove\fP and \fBbelow\fP center the text
+one half line space in the given direction.
+.PP
+Text attributes can be combined:
+.KS
+.PS
+[line up "ljust text" ljust;] 
+move 1.5;
+[line up "rjust text" rjust;] 
+move;
+[arrow 1 "ljust above" ljust above;]
+move;
+[arrow 1 "rjust below" rjust below;]
+.PE
+.CE "1: Text attributes"
+.PP
+What actually happens is that n text strings are centered in a box
+that is \fBtextwid\fP wide by \fBtextht\fP high.  Both these variables
+are initially zero (that is \fBpic\fR's way of not making assumptions
+about \fI[tg]roff\fP(1)'s default point size).
+.PP
+In GNU \fBgpic\fR, objects can have an
+.B aligned
+attribute.
+This will only work when the postprocessor is
+\fBgrops\fP.
+Any text associated with an object having the
+.B aligned
+attribute will be rotated about the center of the object
+so that it is aligned in the direction from the start point
+to the end point of the object.
+Note that this attribute will have no effect for objects whose start and
+end points are coincident.
+.NH 1
+More About Direction Changes
+.PP
+We've already seen how to change the direction in which objects are
+composed from rightwards to downwards.  Here are some more
+illustrative examples:
+.KS
+.PS
+down;
+[
+	"\fBright; box; arrow; circle; arrow; ellipse\fP";
+	move 0.2;
+	[right; box; arrow; circle; arrow; ellipse;]
+]
+move down 0.3 from last [] .s;
+[
+	"\fBleft; box; arrow; circle; arrow; ellipse\fP"
+	move 0.2;
+	[left; box; arrow; circle; arrow; ellipse;]
+]
+# move down 0.3 from last [] .sw;
+# To re-join this illustrations, delete everything from here down to
+# the next #-comment, and uncomment the move line above
+.PE
+.CE "1: Effects of different motion directions (right and left)"
+.KS
+.PS
+# To re-join this illustrations, delete everything down to here, then
+# comment out the next `down' line.
+# Don't forget to re-number the figures following!
+down;
+[
+	"\fBdown; box; arrow; circle; arrow; ellipse;\fP"
+	move 0.2;
+	box; arrow; circle; arrow; ellipse;
+]
+move right 2 from last [] .e;
+[
+	up; box; arrow; circle; arrow; ellipse;
+	move 0.2;
+	"\fBup; box; arrow; circle; arrow; ellipse;\fP"
+]
+.PE
+.CE "2: Effects of different motion directions (up and down)"
+.PP
+Something that may appear surprising happens if you change directions 
+in the obvious way:
+.KS
+.PS
+box; arrow; circle; down; arrow; ellipse
+.PE
+.CE "3: \fBbox; arrow; circle; down; arrow; ellipse\fP
+.LP
+You might have expected that program to yield this:
+.KS
+.PS
+box; arrow; circle; move to last circle .s; down; arrow; ellipse
+.PE
+.CE "4: More intuitive?
+.LP
+But, in fact, to get Figure \*[SN]3 you have to do this:
+.KS
+.DS
+.CW
+\&.PS
+box;
+arrow;
+circle;
+move to last circle .s;
+down;
+arrow;
+ellipse
+\&.PE
+.R
+.DE
+.KE
+Why is this?  Because the exit point for the current direction is
+already set when you draw the object.  The second arrow in Figure
+\*[SN]2 dropped downwards from the circle's attachment point for an
+object to be joined to the right.
+.PP
+The meaning of the command \fBmove to last circle .s\fP should be obvious.
+In order to see how it generalizes, we'll need to go into detail on two
+important topics; locations and object names.
+.NH 1
+Naming Objects
+.PP
+The most natural way to name locations in \fBpic\fP is relative to
+objects.  In order to do this, you have to be able you have to be able
+to name objects.  The \fBpic\fP language has rich facilities for this
+that try to emulate the syntax of English.
+.NH 2
+Naming Objects By Order Of Drawing
+.PP
+The simplest (and generally the most useful) way to name an object is
+with a \fBlast\fP clause.  It needs to be followed by an object type
+name; \fBbox\fP, \fBcircle\fP, \fBellipse\fP, \fBline\fP, \fBarrow\fP,
+\fBspline\fP or \fB[]\fP (the last type refers to a \fIcomposite
+object\fP which we'll discuss later).  So, for example, the \fBlast
+circle\fP clause in the program attached to Figure \*[SN]3 refers to the
+last circle drawn.
+.PP
+More generally, objects of a given type are implicitly numbered
+(starting from 1).  You can refer to (say) the third ellipse in the
+current picture with \fB3rd ellipse\fP, or to the first box as \fB1st
+box\fP, or to the fifth line as \fB5th line\fP.
+.PP
+Objects are also numbered backwards by type from the last one of 
+You can say \fB2nd last box\fP to get the second-to-last box, or
+\fB3rd last ellipse\fP to get the third-to-last box.
+.PP
+In places where \fIn\fBth\fP is allowed, \fB`\fIexpr\fB'th\fP is also allowed.
+Note that
+.B 'th
+is a single token: no space is allowed between the
+.B '
+and the \fBth\fP.
+For example,
+.IP
+.KS
+.R
+.DS
+for i = 1 to 4 do {
+   line from `i'th box.nw to `i+1'th box.se
+}
+.R
+.DE
+.KE
+.NH 2
+Naming Objects With Labels
+.PP
+You can also specify an object by referring to a label.  A label is a
+word (which must begin with a capital letter) followed by a colon;
+you declare it by placing it immediately before the object drawing command.
+For example, the program
+.KS
+.DS
+.CW
+\&.PS
+A: box "first" "object"
+move;
+B: ellipse "second" "object"
+move;
+arrow left at A;
+\&.PE
+.R
+.DE
+.KE
+.LP
+declares labels \fBA\fP and \fBB\fP for its first and second objects.
+Here's what that looks like:
+.KS
+.PS
+A: box "first" "object"
+move;
+B: ellipse "second" "object"
+move;
+arrow left at A .l;
+.PE
+.CE "1: Example of label use"
+The \fBat\fP statement in the fourth line uses the label \fBA\fP (the
+behavior of \fBat\fP will be explained in the next section).  We'll
+see later on that labels are most useful for referring to block composite
+objects.  
+.PP
+Labels are not constants but variables (you can view colon as a sort
+of assignment).  You can say something like \fBA: A + (1,0);\fP
+and the effect will be to reassign the label \fBA\fR to designate a
+position one inch to the right of its old value. 
+.NH 1
+Describing locations
+.PP
+The location of points can be described in many different ways.  All these
+forms are interchangeable as for as the \fBpic\fP language syntax is
+concerned; where you can use one, any of the others that would make 
+semantic sense are allowed.
+.PP
+The special label \fBHere\fR always refers to the current position.
+.NH 2
+Absolute Coordinates
+.PP
+The simplest is absolute coordinates in inches; \fBpic\fP uses a
+Cartesian system with (0, 0) at the lower left corner of the virtual
+drawing surface for each picture (that is, X increases to the right
+and Y increases upwards).  An absolute location may always be written in the
+conventional form as two comma-separated numbers surrounded by
+parentheses (and this is recommended for clarity).  In contexts where
+it creates no ambiguity, the pair of X and Y coordinates suffices
+without punctuation.
+.PP
+It is a good idea to avoid absolute coordinates, however.  They tend
+to make picture descriptions difficult to understand and modify.
+Instead, there are quite a number of ways to specify locations
+relative to \fBpic\fP objects and previous locations.
+.PP
+.NH 2
+Locations Relative to Objects
+.PP
+The symbol \fBhere\fP always refers to the position of the last object
+drawn or the destination of the last \fBmove\fP.
+.PP
+Alone and unqualified, a \fBlast circle\fP or any other way of
+specifying a closed-object or arc location refers as a position to the
+geometric center of the object.  Unqualified, the name of a line or
+spline object refers to the position of the object start.
+.PP
+Also, \fBpic\fP objects have quite a few named locations
+associated with them.  One of these is the object center, which can be
+indicated (redundantly) with the suffix \fB.center\fP (or just \fB.c\fP).
+Thus, \fBlast circle \&.center\fP is equivalent to \fBlast
+circle\fP.
+.NH 3
+Locations Relative to Closed Objects
+.PP
+Every closed object (box, circle, ellipse, or block composite) also
+has eight compass points associated with it;
+.KS
+.PS
+define dot {circle fill rad 0.02 at $1}
+
+define compass { [
+	ME: $1;
+	dot(ME.c);  "\fB .c\fP"  at ME .c ljust;
+	dot(ME.n);  "\fB.n\fP"   at ME .n above
+	dot(ME.ne); "\fB .ne\fP" at ME .ne above
+	dot(ME.e);  "\fB .e\fP"  at ME .e ljust
+	dot(ME.se); "\fB .se\fP" at ME .se below
+	dot(ME.s);  "\fB.s\fP"   at ME .s below
+	dot(ME.sw); "\fB.sw \fP" at ME .sw below
+	dot(ME.w);  "\fB.w \fP"  at ME .w rjust
+	dot(ME.nw); "\fB.nw \fP" at ME .nw above
+] }
+compass(box wid 1.5 ht 1);
+move right from last [] .e;
+compass(circle diam 1);
+move right from last [] .e;
+compass(ellipse wid 1.5 ht 1);
+.PE
+.CE "1: Compass points"
+.LP
+these are the locations where eight compass rays from the geometric center
+would intersect the figure.  So when we say \fBlast circle .s\fP we are
+referring to the south compass point of the last circle drawn.  The
+explanation of Figure 7.3's program is now complete.
+.PP
+(In case you dislike compass points, the names \fB.top\fP,
+\&\fB.bottom\fP, \fB.left\fP and \fB.right\fP are synonyms for \fB.n\fP,
+\&\fB.s\fP, \fB.e\fP, and \fB.w\fP respectively; they can even be
+abbreviated to \fB.t\fP, \fB.b\fP, \fB.l\fP and \fB.r\fP).
+.PP
+The names \fBcenter\fP, \fBtop\fP, \fBbottom\fP, \fBleft\fP and
+\fBright\fP can also be used (without the leading dot) in a prefix
+form marked by \fBof\fP; thus, \fPcenter of last circle\fB and
+\fBtop of 2nd last ellipse\fP are both valid object references.
+.PP
+Arc objects also have compass point; they are the compass points of
+the implied circle.
+.NH 3
+Locations Relative to Open Objects
+.PP
+Every open object (line, arrow, arc, or spline) has three named
+points; \fB.start\fP, \fB.center\fP, and \fB.end\fP.  They can
+also be used without leading dots in the \fBof\fP prefix form.
+The center of an arc is the center of its circle, but the center of
+a line, path, or spline is halfway between its endpoints.
+.KS
+.PS
+define critical {
+	[ ME: $1;
+		dot(ME.c);     ".center" rjust at ME.center + (-0.1, 0.1)
+		dot(ME.start); ".start"  rjust at ME.start  + (-0.1, 0.1)
+		dot(ME.end);   ".end"    rjust at ME.end    + (-0.1, 0.1)
+	]
+	move down 0.2 from last [] .s;
+}
+critical(line up right 1);
+move right from last [] .e;
+critical(arc rad 0.5 cw);
+move right from last [] .e;
+critical(line right 1 then down .5 left 1 then right 1);
+move right from last [] .e;
+critical(spline right 1 then up right then left then left 1);
+.PE
+.CE "2: Special points on open objects"
+.PP
+.NH 2
+Ways of Composing Positions
+.PP
+Once you have two positions to work with, there are several ways to
+combine them to specify new positions.
+.NH 3
+Vector Sums and Displacements
+.PP
+Any two positions may be added or subtracted to yield a new position.
+The result is the conventional vector sum or difference of coordinates.
+For example, \fBlast box .ne + (0.1, 0)\fP is a valid position.  This 
+example illustrates a common use, to define a position slightly offset
+from a named one (say, for captioning purposes).
+.NH 3
+Interpolation Between Positions
+.PP
+A position may be interpolated between any two positions.  The syntax
+is `\fIfraction\fP \fBof the way between\fP \fIposition1\fP \fBand\fP
+\fIposition2\fP.'  For example, you can say \fB1/3 of the way between
+here and last ellipse .ne\fP.  The fraction may be in
+numerator/denominator form or may be an ordinary number (values are
+\&\fInot\fP restricted to [0,1]).  As an alternative to this verbose
+syntax, you can say `\fIfraction\fP  \fB<\fP\fIposition1\fP \fB,\fP
+\fIposition2\fP\fB>\fP.'; thus, the example could also be written
+\fB1/3\fP <here, last ellipse>\fP.
+.KS
+.PS
+arrow up right;
+P: 1/3 of the way between last arrow .start and last arrow .end;
+dot(P); move right 0.1; "P";
+.PE
+.CE "3: \fBP: 1/3 of the way between last arrow .start and last arrow .end\fP"
+.PP
+This facility can be used, for example, to double connections.
+.KS
+.PS
+A: box "yin"; move; B: box "yang";
+arrow right at 1/4 <A.e,A.ne>;
+arrow left  at 1/4 <B.w,B.sw>;
+.PE
+.CE "4: Doubled arrows"
+.LP
+You can get Figure \n[H1]-4 from the following program:
+.KS
+.DS
+.CW
+\&.PS
+A: box "yin"; move;
+B: box "yang";
+arrow right at 1/4 <A.e,A.ne>;
+arrow left  at 1/4 <B.w,B.sw>;
+\&.PE
+.R
+.DE
+.KE
+.LP
+Note the use of the short form for interpolating points.
+.NH 3
+Projections of Points
+.PP
+Given two positions \fIp\fP and \fIq\fP, the position
+\fB(\fP\fIp\fP\fB,\fP \fIq\fP\fB)\fP has the X coordinate of \fIp\fP
+and the Y coordinate of \fIq\fP.  This can be helpful in placing an
+object at one of the corners of the virtual box defined by two other
+objects.
+.KS
+.PS
+box invis wid 2 height 1;
+dot(last box .ne); "\fB(B,A)\fB is here" ljust at last circle + (0.1, 0.1);
+dot(last box .se); "B" ljust at last circle + (0.1, -0.1)
+dot(last box .sw); "\fB(A,B)\fB is here" rjust at last circle + (-0.1, -0.1);
+dot(last box .nw); "A" ljust at last circle + (-0.1, 0.1)
+.PE
+.CE "5: Using (\fIx\fP, \fIy\fP) composition"
+.NH 2
+Using Locations
+.PP
+There are four ways to use locations; \fBat\fP, \fBfrom\fP, \fBto\fP,
+and \fBwith\fP.  All three are object modifiers; that is, you use them
+as suffixes to a drawing command.
+.PP
+The \fBat\fP modifier says to draw a closed object or arc with its
+center at the following location, or to draw a line/spline/arrow
+starting at the following location.
+.PP
+The \fBto\fP modifier can be used alone to specify a move destination.
+The \fBfrom\fP modifier can be used alone in the same way as \fBat\fP.
+.PP
+The \fBfrom\fP and \fBto\fP modifiers can be used with a \fBline\fR or
+\fBarc\fR command to specify start and end points of the object.  In
+conjunction with named locations, this offers a very flexible
+mechanism for connecting objects.  For example, the following program
+.KS
+.DS
+.CW
+\&.PS
+box "from"
+move 0.75;
+ellipse "to"
+arc cw from 1/3 of the way \\
+    between last box .n and last box .ne to last ellipse .n;
+\&.PE
+.R
+.DE
+.KE
+.LP
+yields:
+.KS
+.PS
+box "from"
+move 0.75;
+ellipse "to"
+arc cw from 1/3 of the way between last box .n and last box .ne to last ellipse .n;
+.PE
+.CE "6: A tricky connection specified with English-like syntax"
+.PP
+The \fBwith\fP modifier allows you to identify a named attachment
+point of an object with another point.  This is very useful for connecting
+objects in a natural way.  For an example, consider these two programs:
+.KS
+.PS
+[
+	box wid 0.5 ht 0.5; box wid 0.75 ht 0.75;
+	move down from last box .s 0.1;
+	"\fBbox wid 0.5 ht 0.5; box wid 0.75 ht 0.75\fP"
+]
+move from last [].e 2
+[
+	box wid 0.5 ht 0.5; box wid 0.75 ht 0.75 with .sw at last box .se;
+	move down from last box .s 0.1;
+	"\fBbox wid 0.5 ht 0.5; box wid 0.75 ht 0.75 with .sw at last box .se;\fP"
+]
+.PE
+.CE "7: Using the \fBwith\fP modifier for attachments"
+.NH 2
+The chop modifier
+.PP
+When drawing lines between circles that don't intersect them at a
+compass point, it is useful to be able to shorten a line by the radius 
+of the circle at either or both ends.  Consider the following program:
+.KS
+.DS
+\&.PS
+circle "x"
+circle "y" at 1st circle - (0.4, 0.6) 
+circle "z" at 1st circle + (0.4, -0.6) 
+arrow from 1st circle to 2nd circle chop 
+arrow from 2nd circle to 3rd circle chop 
+arrow from 3rd circle to 1st circle chop 
+\&.PE
+.DE
+.KE
+.LP
+It yields the following:
+.KS
+.PS
+circle "x"
+circle "y" at 1st circle - (0.4, 0.6) 
+circle "z" at 1st circle + (0.4, -0.6) 
+arrow from 1st circle to 2nd circle chop 
+arrow from 2nd circle to 3rd circle chop 
+arrow from 3rd circle to 1st circle chop 
+.PE
+.CE "8: The \fBchop\fR modifier"
+.LP
+Notice that the \fBchop\fR attribute moves arrowheads rather than
+stepping on them. By default, the \fBchop\fR modifier shortens both
+ends of the line by \fBcirclerad\fR.  By suffixing it with a number
+you can change the amount of chopping.
+.PP
+If you say \fBline ... chop \fIr1\fP chop \fIr2\fP\fR with \fIr1\fP
+and \fIr2\fP both numbers, you can vary the amount of chopping at both
+ends.  You can use this in combination with trigonometric functions
+to write code that will deal with more complex intersections.
+.NH 1
+Object Groups
+.PP
+There are two different ways to group objects in \fBpic\fP; \fIbrace
+grouping\fP and \fIblock composites\fP.
+.NH 2
+Brace Grouping
+.PP
+The simpler method is simply to group a set of objects within curly
+bracket or brace characters.  On exit from this grouping, the current
+position and direction are restored to their value when the opening
+brace was encountered.
+.NH 2
+Block Composites
+.PP
+A block composite object is created a series of commands enclosed by
+square brackets.  The composite can be treated for most purposes like
+a single closed object, with the size and shape of its bounding box.
+Here is an example.  The program fragment
+.KS
+.DS
+.CW
+\&A: [
+\&    circle;
+\&    line up 1 at last circle .n;
+\&    line down 1 at last circle .s;
+\&    line right 1 at last circle .e;
+\&    line left 1 at last circle .w;
+\&    box dashed with .nw at last circle .se + (0.2, -0.2); 
+\&    Caption: center of last box;
+\&]
+.R
+.DE
+.KE
+.LP
+yields the block in figure \n[H1]-1, which we show both with and
+without its attachment points.  The block's location becomes the
+value of \fBA\fP.
+.KS
+.PS
+define junction {
+	circle;
+	line up 1 at last circle .n;
+	line down 1 at last circle .s;
+	line right 1 at last circle .e;
+	line left 1 at last circle .w;
+	box dashed with .nw at last circle .se + (0.2, -0.2); 
+	Caption: center of last box;
+}
+[junction();]
+move;
+compass([junction()]);
+.PE
+.CE "1: A sample composite object"
+.LP
+To refer to one of the composite's attachment points, you can say
+(for example) \fBA .s\fP.  For purposes of object naming, composites
+are a class.  You could write \fBlast [] .s\fP as an equivalent
+refrence, usable anywhere a location is needed.  This construction is
+very important for putting together large, multi-part diagrams.
+.PP 
+Blocks are also a variable-scoping mechanism, like a \fIgroff\fP(1)
+environment.  All variable assignments done inside a block are undone
+at the end of it.  To get at values within a block, write a name of
+the block followed by a dot, followed by the variable or label you
+want.  For example, we could refer the the center of the box in the
+above composite as \fBlast [] .Caption\fP or \fBA.Caption\fP.
+.PP
+This kind of reference to a label can be used in any way any other
+location can be.  For example, if we added \fB"Hi!" at A.Caption\fP
+the result would look like this:
+.KS
+.PS
+A: [junction();]
+"Hi!" at A.Caption;
+.PE
+.CE "2: Adding a caption using interior labeling"
+.PP
+You can also use interior labels in either part of a \fBwith\fR
+modifier.  This means that the example composite could be placed
+relative to its caption box by a command containing \fBwith A.Caption
+at\fP.
+.PP
+Blocks may be nested.  This means you can use block attachment points
+to build up complex diagrams hierarchically, from the inside out.
+Note that \fBlast\fP and the other sequential naming mechanisms
+don't look inside blocks, so if you have a program that looks 
+like 
+.KS
+.DS
+.CW
+\&.PS
+P: [box "foo"; ellipse "bar"];
+Q: [
+	[box "baz"; ellipse "quxx"]
+	"random text";
+   ]
+arrow from 2nd last [];
+\&.PE
+.R
+.DE
+.KE
+.LP
+the arrow in the last line will be attached to object \fBP\fP, not 
+object \fBQ\fP.
+.PP
+In DWB \fBpic\fP, only references one level deep into enclosed blocks
+were permitted. GNU \fBgpic\fP removes this restriction.
+.PP
+The combination of block variable scoping, assignability of labels and
+the macro facility that we'll describe later on can be used to
+simulate functions with local variables (just wrap the macro body in
+block braces).
+.NH 1
+Style Variables
+.PP
+There are a number of global style variables in \fBpic\fR that can be used to
+change its overall behavior.  We've mentioned several of them in
+previous sections.  They're all described here.  For each variable,
+the default is given.
+.TS H
+tab(@), linesize(2);
+lb | lb | lb
+l | n | l.
+.sp 2p
+Style Variable@Default@What It Does
+.sp 2p
+_
+.sp 2p
+.TH
+boxht@0.5@Default height of a box
+boxwid@0.75@Default height of a box
+lineht@0.5@Default length of vertical line
+linewid@0.75@Default length of horizontal line
+arcrad @0.25@Default radius of an arc
+circlerad@0.25@Default radius of a circle
+ellipseht@0.5@Default height of an ellipse
+ellipsewid@0.75@Default width of an ellipse
+moveht@0.5@Default length of vertical move
+movewid@0.75@Default length of horizontal move
+textht@0@Default height of box enclosing a text object
+textwid@0@Default width of box enclosing a text object
+arrowht@0.1@Length of arrowhead along shaft
+arrowwid@0.05@Width of rear of arrowhead
+arrowhead@1@Enable/disable arrowhead filling
+dashwid@0.05@Interval for dashed lines
+maxpswid@11@Maximum width of picture
+maxpsht@8.5@Maximum height of picture
+scale@1@Unit scale factor
+fillval@0.5@Default fill value
+.sp 5p
+_
+.TE
+Any of these variables can be set with a simple assignment statement.
+For example:
+.KS
+.PS
+[boxht=1; boxwid=0.3; movewid=0.2; box; move; box; move; box; move; box;]
+.PE
+.CE "1: \fBboxht=1; boxwid=0.3; movewid=0.2; box; move; box; move; box; move; box;\fP"
+.PP
+In GNU \fBpic\fR, setting the \fBscale\fR variable re-scales all
+size-related state variables so that their values remain equivalent in
+the new units.
+.PP
+The command \fBreset\fP resets all style variables to their defaults.
+You can give it a comma-separated list of variable names as arguments,
+in which case it resets only those.
+.PP
+State variables retain their values across pictures until reset.
+.NH 1
+Expressions, Variables, and Assignment
+.PP
+A number is a valid expression, of course (all numbers are stored
+internally as floating-point).  Decimal-point notation is acceptable;
+in GNU \fBgpic\fR, scientific notation in C's `e' format (like
+\f(CW5e-2\fP) is accepted.
+.PP
+Anywhere a number is expected, the language will also accept a
+variable.  Variables may be the built-in style variable described in
+the last section, or new variables created by assignment.
+.PP
+DWB \fBpic\fP supports only the ordinary assignment via =, defines the
+variable in the current block if it is not already defined there, and
+then changes the value in the current block.
+GNU \fBgpic\fP supports an alternate form of assignment using :=. The
+.I variable
+(right side) must already be defined,
+and the value of
+.I variable
+will be changed only in the innermost block in which it is defined.
+.PP
+You can use the height, width, radius, and x and y coordinates of any
+object or corner in expressions If \fBA\fP is an object label or name,
+all the following are valid:
+.KS
+.DS
+.CW
+A.x                  # x coordinate of the center of A
+A.ne.y               # y coordinate of the northeast corner of A
+A.wid                # the width of A
+A.ht                 # and its height
+2nd last circle.rad  # the radius of the 2nd last circle
+.R
+.DE
+.KE
+Note the second expression, showing how to extract a corner coordinate.
+.PP
+Basic arithmetic resembling those of C operators are available; +, *,
+-, /, and %.  So is ^ for exponentiation.  Grouping is permitted in
+the usual way using parentheses.  GNU \fBgpic\fP allows logical
+operators to appear in expressions; ! (logical negation, not
+factorial), &&, ||, ==, !=, >=, <=, <, >.
+.PP
+Various built-in functions are supported: \fBsin(\fIx\fP)\fP,
+\fBcos(\fIx\fP)\fP, \fBlog(\fIx\fP)\fP, \fBexp(\fIx\fP)\fP,
+\fBsqrt(\fIx\fP)\fP, \fBmax(\fIx\fP,\fIy\fP)\fP,
+\fBatan2(\fIx\fP,\fIy\fP)\fP, \fBmin(\fIx\fP,\fIy\fP)\fP,
+\fBint(\fIx\fP\fB)\fP, and \fBrand()\fP, Both \fBexp\fP and \fBlog\fP are
+base 10; \fBint\fP does integer truncation; and \fBrand()\fP returns a
+random number in [0-1).
+.PP
+GNU \fBgpic\fP also documents a one-argument form or rand,
+\fBrand(\fIx\fP\fB)\fP, which returns a random number between 1 and
+\fIx\fP, but this is deprecated and may be removed in a future
+version.
+.PP
+The function \fBsprintf()\fP behaves like a C \fIsprintf\fP(3) that
+only takes %, %e, %f, and %g format strings.
+.NH 1
+Macros
+.PP
+You can define macros in \fBpic\fP.  This is useful for diagrams with
+repetitive parts.  In conjunction with the scope rules for block
+composites, it effectively gives you the ability to write functions.
+.PP
+The syntax is
+.DS
+.CW
+\fBdefine\fP \fIname\fP \fB{\fP \fIreplacement text \fB}\fP
+.R
+.DE
+.LP
+This defines \fIname\fR as a macro to be replaced by the replacement
+text (not including the braces).  The macro may be called as 
+.DS
+.CW
+\fIname\fB(\fIarg1, arg2, ... argn\fB)\fR
+.R
+.DE
+.LP
+The arguments (if any) will be substituted for tokens $1, $2 ... $n
+appearing in the replacement text.
+.PP
+As an example of macro use, consider the following:
+.KS
+.DS
+.CW
+\&.PS
+\&# Plot a single jumper in a $1 by $2 box, $3 is the on-off state
+\&define jumper { [
+\&    shrinkfactor = 0.8;
+\&    Outer: box invis wid 0.5 ht 1;
+\&
+\&    # Count on end ] to reset these
+\&    boxwid = Outer.wid * shrinkfactor / 2;
+\&    boxht  = Outer.ht  * shrinkfactor / 2;
+\&
+\&    box fill (!$1) with .s at center of Outer;
+\&    box fill ($1)  with .n at center of Outer;
+\&] }
+\&
+\&# Plot a block of six jumpers
+\&define jumperblock {
+\&    jumper($1);
+\&    jumper($2);
+\&    jumper($3);
+\&    jumper($4);
+\&    jumper($5);
+\&    jumper($6);
+\&
+\&    jwidth  = last [].Outer.wid;
+\&    jheight = last [].Outer.ht;
+\&
+\&    box with .nw at 6th last [].nw wid 6*jwidth ht jheight;
+\&
+\&    # Use {} to avoid changing position from last box draw.
+\&    # This is necessary so move in any direction will work as expected
+\&    {"Jumpers in state $1$2$2$3$4$5$6" at last box .s + (0, -0.2);}
+\&}
+\&
+\&# Sample macro invocations
+\&jumperblock(1,1,0,0,1,0);
+\&move;
+\&jumperblock(1,0,1,0,1,1);
+.PE
+.R
+.DE
+.KE
+It yields the following:
+.KS
+.PS
+# Plot a single jumper in a $1 by $2 box, $3 is the on-off state
+define jumper { [
+    shrinkfactor = 0.8;
+    Outer: box invis wid 0.5 ht 1;
+
+    # Count on end ] to reset these
+    boxwid = Outer.wid * shrinkfactor / 2;
+    boxht  = Outer.ht  * shrinkfactor / 2;
+
+    box fill (!$1) with .s at center of Outer;
+    box fill ($1)  with .n at center of Outer;
+] }
+
+# Plot a block of six jumpers
+define jumperblock {
+    jumper($1);
+    jumper($2);
+    jumper($3);
+    jumper($4);
+    jumper($5);
+    jumper($6);
+
+    jwidth  = last [].Outer.wid;
+    jheight = last [].Outer.ht;
+
+    box with .nw at 6th last [].nw wid 6*jwidth ht jheight;
+
+    # Use {} to avoid changing position from last box draw.
+    # This is necessary so move in any direction will work as expected
+    {"Jumpers in state $1$2$2$3$4$5$6" at last box .s + (0, -0.2);}
+}
+
+# Sample macro invocations
+jumperblock(1,1,0,0,1,0);
+move;
+jumperblock(1,0,1,0,1,1);
+.PE
+.CE "1: Sample use of a macro"
+.LP
+This macro example illustrates how you can combine [], brace grouping,
+and variable assignment to write true functions.
+.PP
+One detail the example above does not illustrate is the fact that
+macro argument parsing is not token-oriented.  If you call \fBjumper(
+1 )\fP, the value of $1 will be \fB" 1 "\fP. You could even call
+\fBjumper(big  string)\fP to give $1 the value \fB"big  string"\fP.
+.PP
+If you want to pass in a coordinate pair, you can avoid getting
+tripped up by the comma by wrapping the pair in parentheses.
+.PP
+Macros persist through pictures.  To undefine a mcro, say \fBundef\fP
+\fIname\fR; for example,
+.DS
+\f(CWundef jumper\fP
+\f(CWundef jumperblock\fP
+.DE
+.LP
+would undefine the two macros in the jumper block example.
+.NH 1
+Import/Export Commands
+.PP
+Commands that import or export data between \fBpic\fR and its
+environment are described here.
+.NH 2
+File and Table Insertion
+.PP
+The statement
+.DS
+\f(CWcopy\fP \fIfilename\fR
+.DE
+inserts the contents of \fIfilename\fR in the \fBpic\fP input stream.
+Any .PS/.PE pair in the file will be ignored.  This, you can use this
+to include pre-generated images.
+.PP
+A variant of this statement replicates the \fBcopy thru\fP feature of
+\fIgrap\fP(1).  If you say
+.DS
+\f(CWcopy\fP \fIfilename\fR \f(CWthru\fP \fImacro\fP
+.DE
+.LP
+calls the \fImacro\fP (which may be either a name or replacement text)
+on the arguments obtained by breaking each line of the file into
+blank-separated fields.  The macro may have up to 9 arguments.  The
+replacement text may be delimited by braces or by a pair of instances
+of any character not appearing in the rest of the text.
+.PP
+If you write
+.DS
+\f(CWcopy thru\fP \fImacro\fP
+.DE
+.LP
+omitting the filename, lines to be parsed are taken from the input
+source up to the next .PE.
+.PP
+In either of the \fBcopy\fP commands, GNU \fBgpic\fP permits a
+trailing `\fBuntil\fP \fIword\fP' clause to be added which terminates
+the copy when the first word matches the argument (the default
+behavior is therefore equivalent to \fBuntil .PE\fP,
+.PP
+Accordingly, the command
+.RS
+.IP
+.ft CW
+.nf
+\&.PS
+copy thru % circle at ($1,$2) % until "END"
+1 2
+3 4
+5 6
+END
+box
+\&.PE
+.R
+.fi
+.RE
+.LP
+is equivalent to
+.RS
+.IP
+.ft CW
+.nf
+\&.PS
+circle at (1,2)
+circle at (3,4)
+circle at (5,6)
+box
+\&.PE
+.R
+.fi
+.RE
+.NH 2
+Debug Messages
+.PP
+The command \fBprint\fR accepts any number of comma-separated
+arguments, concatenates their output forms, and writes the result to
+standard error.  Each argument must be an expression, a position, or
+a text string.
+.NH 2
+Escape to Post-Processor
+.PP
+If you write
+.DS
+\fBcommand\fR \fIarg\fR\|.\|.\|.
+.DE
+.LP
+\fBpic\fP concatenates the arguments and pass them through as a line
+to troff or \*(tx.  Each
+.I arg
+must be an expression, a position, or text.
+This has a similar effect to a line beginning with
+.B .
+or
+\fB\e\fR,
+but allows the values of variables to be passed through.
+.NH 2
+Executing Shell Commands
+.PP
+The command 
+.DS
+\f(CWsh { \fIanything...\fP }\fP
+.DE
+.LP
+macroexpands the text in braces, then executes it as a shell command.
+This could be used to generate images or data tables for later
+inclusion.  The delimiters shown as {} here may also be two copies of
+any one character not present in the shell command text.  In either
+case, the body may contain balanced {} pairs.  Strings in the body
+may contain balanced or unbalanced braces in any case.
+.NH 1
+Control-flow constructs
+.PP
+The \fBpic\fP language provides conditionals and looping.  For
+example,
+.KS
+.DS
+.CW
+pi = atan2(0, -1);
+for i = 0 to 2 * pi by 0.1 do {
+    "-" at (i/2, 0);
+    "." at (i/2, sin(i)/2);
+    ":" at (i/2, cos(i)/2);
+}
+.R
+.DE
+.KE
+.LP
+which yields this:
+.KS
+.PS
+pi = atan2(0, -1);
+for i = 0 to 2 * pi by 0.1 do {
+    "-" at (i/2, 0);
+    "." at (i/2, sin(i)/2);
+    ":" at (i/2, cos(i)/2);
+}
+.PE
+.CE "1: Plotting with a \fBfor\fP loop"
+.LP
+The syntax of the \fBfor\fP statement is:
+.DS
+\fBfor\fR \fIvariable\fR \fB=\fR \fIexpr1\fR \fBto\fR \fIexpr2\fR \
+[\fBby\fR [\fB*\fR]\fIexpr3\fR] \fBdo\fR \fIX\fR \fIbody\fR \fIX\fR
+.DE
+The semantics are as follows:  Set
+.I variable
+to \fIexpr\fR1 .
+While the value of
+.I variable
+is less than or equal to
+\fIexpr2\fR,
+do
+.I body
+and increment
+.I variable
+by
+\fIexpr3\fR;
+if
+.B by
+is not given, increment
+.I variable
+by 1.
+If
+.I expr3
+is prefixed by
+.B *
+then
+.I variable
+will instead be multiplied by
+\fIexpr3\fR.
+.I X
+can be any character not occurring in
+\fIbody\fR; or the two Xs may be paired braces (as in the \fBsh\fR command).
+.PP
+The syntax of the \fBif\fP statement is as follows:
+.DS
+\fBif\fR \fIexpr\fR \fBthen\fR \fIX\fR \fIif-true\fR \fIX\fR \
+[\fBelse\fR \fIY\fR \fIif-false\fR \fIY\fR]
+.DE
+Its semantics are as follows: Evaluate
+\fIexpr\fR;
+if it is non-zero then do
+\fIif-true\fR,
+otherwise do
+\fIif-false\fR.
+.I X
+can be any character not occurring in
+\fIif-true\fR.
+.I Y
+can be any character not occurring in
+\fIif-false\fR.
+.PP
+Eithe or both of the X or Y pairs may instead be balanced pairs of
+braces ({ and }) as in the \fBsh\fR command.  In either case, the
+\fIif-true\fR may contain balanced pairs of braces.  None of these
+delimiters will be seen inside strings.
+.PP
+All the usual relational operators my be used in conditional expressions;
+! (logical negation, not factorial), &&, ||, ==, !=, >=, <=, <, >.
+.PP
+String comparison is also supported using == and !=.  String
+comparisons may need to be parenthesized to avoid syntactic
+ambiguities.
+.NH 1
+Interface To [gt]roff
+.PP
+The output of \fBpic\fP is \fB[gt]roff\fP drawing commands.  The GNU
+\fIgpic\fP(1) command warns that it relies on drawing extensions
+present in \fIgroff\fP(1) that are not present in \fItroff\fP(1).
+.NH 2
+Scaling Arguments
+.PP
+The DWB \fIpic\fP(1) program will accept one or two arguments to
+\&\fB.PS\fP, which is interpreted as a width and height in inches to
+which the results of \fIpic\fP(1) should be scaled (width and height
+scale independently).  If there is only one argument, it is
+interpreted as a width to scale the picture to, and height will be
+scaled by the same proportion.
+.PP
+GNU \fBgpic\fP is less general; it will accept a single width to scale
+to, or a zero width and a maximum height to scale to.  With
+two nonzero arguments, it will scale to the maximum height.
+.NH 2
+How Scaling is Handled
+.PP
+When \fBpic\fP processes a picture description on input, it passes .PS
+and .PE through to the postprocessor. The .PS gets decorated with two
+numeric arguments which are the X and Y dimensions of the picture in
+inches.  The post-processor can use these to reserve space for the
+picture and center it.
+.PP
+The \fBmgs\fP macros, for example, include the following definitions:
+.KS
+.DS
+.CW
+\&.de PS
+\&.br
+\&.sp \\n[DD]u
+\&.ie \\n[.$]<2 .@error bad arguments to PS (not preprocessed with pic?)
+\&.el \{\
+\&.	ds@need (u;\\$1)+1v
+\&.	in +(u;\\n[.l]-\\n[.i]-\\$2/2>?0)
+\&.\}
+\&..
+\&.de PE
+\&.par@reset
+\&.sp \\n[DD]u+.5m
+\&..
+.R
+.DE
+.KE
+.LP
+Equivalent definition will be supplied by GNU \fIpic\fP(1) if you use
+the -mpic option; this should make it usable with macro pages other
+than \fIms\fR(1).
+.PP
+if .PF is used instead of .PE, the \fBtroff\fP position is restored to
+what it was at the picture start (Kernighan notes that the F stands
+for "flyback").
+.PP
+The invocation
+.DS
+\&\fB.PS <\fP\fIfile\fP
+.DE
+.LP
+causes the contents of \fIfile\fP to replace the .PS line.  This
+feature is deprecated; use \fBcopy file\fR instead).
+.PP
+By default, input lines that begin with a period are passed to the
+postprocessor, embedded at the corresponding point in the output.
+Messing with horizontal or vertical spacing is an obvious recipe for
+bugs, but point size and font changes will usually be safe.
+.PP
+Point sizes and font changes are also safe within text strings, as
+long as they are undone before the end of string.
+.PP
+The state of \fB[gt]roff\fP's fill mode is preserved across pictures.
+.PP
+The Kernighan paper notes that there is a subtle problem with
+complicated equations inside \fBpic\fR pictures; they come out wrong if
+\fIeqn\fP(1)  has to leave extra vertical space for the equation.
+If your equation involves more than subscripts and superscripts, you
+must add to the beginning of each equation the extra information
+\f(CWspace 0\fP.  He gives the following example:
+.KS
+.DS
+.CW
+arrow
+box "$space 0 {H( omega )} over {1 - H( omega )}$"
+arrow
+.R
+.DE
+.KE
+.EQ
+delim @@
+.EN
+.KS
+.PS
+arrow
+box "@space 0 {H( omega )} over {1 - H( omega )}@"
+arrow
+.PE
+.CE "1: Equations within pictures"
+.NH 1
+Interface to TeX
+.PP
+.PP
+\*(tx mode is enabled by the
+.B \-t
+option.
+In \*(tx mode, pic will define a vbox called
+.B \egraph
+for each picture.
+You must yourself print that vbox using, for example, the command
+.RS
+.LP
+.B
+\ecenterline{\ebox\egraph}
+.RE
+.LP
+Actually, since the vbox has a height of zero this will produce
+slightly more vertical space above the picture than below it;
+.RS
+.LP
+.B
+\ecenterline{\eraise 1em\ebox\egraph}
+.RE
+.LP
+would avoid this.
+.PP
+You must use a \*(tx driver that supports the
+.B tpic
+specials, version 2.
+.PP
+Lines beginning with
+.B \e
+are passed through transparently; a
+.B %
+is added to the end of the line to avoid unwanted spaces.
+You can safely use this feature to change fonts or to
+change the value of \fI\ebaselineskip\fR.
+Anything else may well produce undesirable results; use at your own risk.
+Lines beginning with a period are not given any special treatment.
+.PP
+The \(*tx mode of \fIpic\fP(1) will \fInot\fP translate \fBtroff\fP
+font and size changes included in text strings!
+.NH 1
+Obsolete Commands
+.PP
+GNU \fIgpic\fP(1)  has a command
+.DS
+\fBplot\fR \fIexpr\fR [\fB"\fItext\fB"\fR]
+.DE
+This is a text object which is constructed by using
+.I text
+as a format string for sprintf
+with an argument of
+\fIexpr\fP.
+If
+.I text
+is omitted a format string of "%g" is used.
+Attributes can be specified in the same way as for a normal text
+object.
+Be very careful that you specify an appropriate format string;
+\fBpic\fP does only very limited checking of the string.
+This is deprecated in favour of
+\fBsprintf\fP.
+.NH 1
+Some Larger Examples
+.PP
+Here are a few larger examples, with complete source code.
+.PP
+One of our earlier examples is generated in an instructive way using a
+for loop:
+.KS
+.DS
+.CW
+\&.PS
+\&# Draw a demonstration up left arrow with grid box overlay
+\&define gridarrow
+\&{
+\&    [
+\&        {arrow up left $1;}
+\&        box wid 0.5 ht 0.5 dotted with .nw at last arrow .end;
+\&        for i = 2 to ($1 / 0.5) do
+\&        {
+\&            box wid 0.5 ht 0.5 dotted with .sw at last box .se;
+\&        }
+\&        move down from last arrow .center;
+\&        [
+\&            if ( $1 == boxht ) then { "\\fBline up left\\fP" } else { sprintf("\\fBarrow up left %g\\fP", $1) };
+\&        ]
+\&    ] 
+\&    move right from last [] .e;
+\&}
+\&gridarrow(0.5);
+\&gridarrow(1);
+\&gridarrow(1.5);
+\&gridarrow(2);
+\&undef gridarrow
+\&.PE
+.R
+.DE
+.KE
+.KS
+.PS
+# Draw a demonstration up left arrow with grid box overlay
+define gridarrow
+{
+	[
+		{arrow up left $1;}
+		box wid 0.5 ht 0.5 dotted with .nw at last arrow .end;
+		for i = 2 to ($1 / 0.5) do
+		{
+			box wid 0.5 ht 0.5 dotted with .sw at last box .se;
+		}
+		move down from last arrow .center;
+		[
+			if ( $1 == boxht ) then { "\fBline up left\fP" } else { sprintf("\fBarrow up left %g\fP", $1) };
+		]
+	] 
+	move right from last [] .e;
+}
+gridarrow(0.5);
+gridarrow(1);
+gridarrow(1.5);
+gridarrow(2);
+undef gridarrow
+.PE
+.CE "1: Diagonal arrows (dotted boxes show the implied 0.5-inch grid)"
+.PP
+Here's an example concocted to demonstrate layout of a large,
+multiple-part pattern:
+.KS
+.DS
+.CW
+\&.PS
+\&define filter {box ht 0.25 rad 0.125}
+\&lineht = 0.25;
+\&Top: [
+\&    right;
+\&    box "\\fBms\\fR" "sources";
+\&    move;
+\&    box "\\fBHTML\\fR" "sources";
+\&    move;
+\&    box "\\fBlinuxdoc-sgml\\fP" "sources" wid 1.5;
+\&    move;
+\&    box "\\fBTexinfo\\fP" "sources";
+\&
+\&    line down from 1st box .s lineht;
+\&    A: line down;
+\&    line down from 2nd box .s; filter "\\fBhtml2ms";
+\&    B: line down;
+\&    line down from 3rd box .s; filter "\\fBformat\\fP";
+\&    C: line down;
+\&    line down from 4th box .s; filter "\\fBtexi2roff\\fP";
+\&    D: line down;
+\&]
+\&move down 1 from last [] .s;
+\&Anchor: box wid 1 ht 0.75 "\\fBms\\fR" "intermediate" "form";
+\&arrow from Top.A.end to Anchor.nw;
+\&arrow from Top.B.end to 1/3 of the way between Anchor.nw and Anchor.ne;
+\&arrow from Top.C.end to 2/3 of the way between Anchor.nw and Anchor.ne;
+\&arrow from Top.D.end to Anchor.ne
+\&{
+\&    # PostScript column
+\&    move to Anchor .sw;
+\&    line down left then down ->;
+\&    filter "\\fBpic\\fP";
+\&    arrow;
+\&    filter "\\fBeqn\\fP";
+\&    arrow;
+\&    filter "\\fBtbl\\fP";
+\&    arrow;
+\&    filter "\\fBgroff\\fP";
+\&    arrow;
+\&    box "PostScript";
+\&
+\&    # HTML column
+\&    move to Anchor .se;
+\&    line down right then down ->;
+\&    A: filter dotted "\\fBpic2img\\fP";
+\&    arrow;
+\&    B: filter dotted "\\fBeqn2html\\fP";
+\&    arrow;
+\&    C: filter dotted "\\fBtbl2html\\fP";
+\&    arrow;
+\&    filter "\\fBms2html\\fP";
+\&    arrow;
+\&    box "HTML";
+\&
+\&    # Nonexistence caption
+\&    box dashed wid 1 at B + (2, 0) "These tools" "don't yet exist";
+\&    line chop 0 chop 0.1 dashed from last box .nw to A.e ->;
+\&    line chop 0 chop 0.1 dashed from last box .w  to B.e ->;
+\&    line chop 0 chop 0.1 dashed from last box .sw to C.e ->;
+\&}
+\&.PE
+.R
+.DE
+.KE
+.KS
+.PS
+define filter {box ht 0.25 rad 0.125}
+lineht = 0.25;
+Top: [
+    right;
+    box "\fBms\fR" "sources";
+    move;
+    box "\fBHTML\fR" "sources";
+    move;
+    box "\fBlinuxdoc-sgml\fP" "sources" wid 1.5;
+    move;
+    box "\fBTexinfo\fP" "sources";
+
+    line down from 1st box .s lineht;
+    A: line down;
+    line down from 2nd box .s; filter "\fBhtml2ms";
+    B: line down;
+    line down from 3rd box .s; filter "\fBformat\fP";
+    C: line down;
+    line down from 4th box .s; filter "\fBtexi2roff\fP";
+    D: line down;
+]
+move down 1 from last [] .s;
+Anchor: box wid 1 ht 0.75 "\fBms\fR" "intermediate" "form";
+arrow from Top.A.end to Anchor.nw;
+arrow from Top.B.end to 1/3 of the way between Anchor.nw and Anchor.ne;
+arrow from Top.C.end to 2/3 of the way between Anchor.nw and Anchor.ne;
+arrow from Top.D.end to Anchor.ne
+{
+    # PostScript column
+    move to Anchor .sw;
+    line down left then down ->;
+    filter "\fBpic\fP";
+    arrow;
+    filter "\fBeqn\fP";
+    arrow;
+    filter "\fBtbl\fP";
+    arrow;
+    filter "\fBgroff\fP";
+    arrow;
+    box "PostScript";
+
+    # HTML column
+    move to Anchor .se;
+    line down right then down ->;
+    A: filter dotted "\fBpic2img\fP";
+    arrow;
+    B: filter dotted "\fBeqn2html\fP";
+    arrow;
+    C: filter dotted "\fBtbl2html\fP";
+    arrow;
+    filter "\fBms2html\fP";
+    arrow;
+    box "HTML";
+
+    # Nonexistence caption
+    box dashed wid 1 at B + (2, 0) "These tools" "don't yet exist";
+    line chop 0 chop 0.1 dashed from last box .nw to A.e ->;
+    line chop 0 chop 0.1 dashed from last box .w  to B.e ->;
+    line chop 0 chop 0.1 dashed from last box .sw to C.e ->;
+}
+.PE
+.CE "2: Hypothetical production flow for dual-mode publishing"
+.LP
+.\"%%REFERENCE%%
+.NH 1
+PIC Reference
+.PP
+This is an annotated grammar of PIC.
+.NH 2
+Lexical Items
+.PP
+In general, \fBpic\fP is a free-format, token-oriented language that
+ignores whitespace outside strings.  But certain lines and contructs
+are specially interpreted at the lexical level:
+.PP
+A comment begins with # and continues to \en (comments may also follow
+text in a line).  A line beginning with a period or backslash may be
+interpreted as text to be passed through to the post-processor,
+depending on command-line options.  An end-of-line backslash is
+interpreted as a request to continue the line; the backslash and
+following newline are ignored.
+.LP
+Here are the grammar terminals:
+.IP <number> 
+A decimal numeric constant.  May contain a decimal point or be
+expressed in scientific notation in the style of \fIprintf\fP(3)'s %e
+escape. (All variables are represented internally in floating-point.)
+.IP <string>
+Any ASCII characters surrounded by a pair of double quotes.  May
+contain a double quote if preceded by a backslash.
+.IP <variable>
+A lower-case alphabetic character, followed by any number of
+alphanumerics.  (Values of variables are preserved across pictures.)
+.IP <label>
+An upper-case alphabetic character, followed by any number of
+alphanumerics.
+.NH 2
+Semi-Formal Grammar
+.PP
+Tokens not enclosed in <> are literals, except:
+.IP 1.
+\en is a newline
+.IP 2.
+three dots is a suffix meaning \&`replace with 0 or more repetitions
+of the preceding element.
+.IP 3.
+enclosure in square brackets has its usual meaning of `this clause is
+optional'.
+.IP 4.
+Square-bracket-enclosed portions within tokens are optional.  Thus,
+\fBh[eigh]t\fR matches either `height' or `ht'.
+.LP
+If one of these special tokens has to be referred to literally, it is
+surrounded with single quotes.
+.PP
+The top-level \fBpic\fP object is a picture.
+.DS
+.CW
+<picture>    ::= .PS [width [height]]\en
+                 <statement> ...
+                 .PE \en
+.R
+.DE
+.LP
+The \fIwidth\fP and \fIheight\fP arguments, if present, cause
+\fBpic\fR to attempt to scale the picture to the given dimensions in
+inches.  In no case, however, will the X and Y dimensions of the
+picture exceed the values of the style variables \fBmaxpswid\fP,
+\fBmaxpsheight\fP (which default to the normal 8.5 by 11 page size)\fP.
+.LP
+If the ending .PS is replaced by .PF, the page vertical position is
+restored to its value at the time .PS was encountered.  Another
+alternate form of invocation is \f(CW.PS <\fIfilename\fR, which
+replaces the .PS line with a file to be interpreted by \fBpic\fR (but
+this feature is deprecated).
+.LP
+The .PS, .PE, and .PF macros to perform centering and scaling are
+normally supplied by the post-processor.
+.DS
+.CW
+
+<statement>  ::= <command> ;
+                 <command> \en
+
+<command>    ::= <primitive> <modifier>...
+                 <label> : <command>
+                 <label> : <position>
+                 <variable> = <expr>
+                 <direction>
+                 { <command> ... }
+                 '[' <command> ... ']'
+                 for <var> = <expr> to <expr> [by <expr>] do { <command> ... }
+                 if <expr> then { <command> ... } [else { <command> ... }]
+                 copy <filename> [until <word>]
+                 copy <filename> thru <macroname> [until <word>] 
+                 sh <balanced-text>
+                 print <print-item>
+                 reset [ <variable> ... ]
+.R
+.DE
+.LP
+The current position and direction are saved on entry to a { } and
+restored on exit from it.
+.LP
+Drawn objects within [ ] are treated as a single composite object with
+a rectangular shape (that of the bounding box of all the elements).
+Variable and label assignments within a block are local to the block.
+Current direction of motion is restored to the value at start of block
+upon exit.  Position is \fInot\fR restored (unlike { }) instead, the
+current position becomes the exit position for the current direction
+on the block's bounding box.
+.DS
+.CW
+<primitive>  ::= box                   \fR# Closed object -- rectangle\fP
+                 circle                \fR# Closed object -- circle\fP
+                 ellipse               \fR# Closed object -- ellipse\fP
+                 arc                   \fR# Open object -- quarter-circle\fP
+                 line                  \fR# Open object -- line\fP
+                 arrow                 \fR# Open object -- line with arrowhead\fP
+                 spline                \fR# Open object -- spline curve\fP
+                 move
+                 <text> <text> ...     \fR# Text within invisible box\fP
+
+<attribute>  ::= h[eigh]t <expr>       \fR# Set height of closed figure \fP
+                 wid[th] <expr>        \fR# Set width of closed figure \fP
+                 rad[ius] <expr>       \fR# Set radius of circle/arc \fP
+                 diam[eter] <expr>     \fR# Set diameter of circle/arc \fP
+                 up [ <expr> ]         \fR# Move up \fP
+                 down [ <expr> ]       \fR# Move down \fP
+                 left [ <expr> ]       \fR# Move left \fP
+                 right [ <expr> ]      \fR# Move right \fP
+                 from <position>       \fR# Set from position of open figure\fP
+                 to <position>         \fR# Set to position of open figure\fP
+                 at <position>         \fR# Set center of open figure\fP
+                 with <corner>         \fR# Fix corner at specified location\fP
+                 by <expr> <expr>      \fR# Set object's attachment point\fP
+                 then                  \fR# Sequential segment composition\fP
+                 dotted [ <expr> ]     \fR# Set dotted line style\fP
+                 dashed [ <expr> ]     \fR# Set dashed line style\fP
+                 chop [ <expr> ]       \fR# Chop end(s) of segment\fP
+                 ->                    \fR# Decorate with "to" arrow\fP
+                 <-                    \fR# Decorate with "from" arrow\fP
+                 <->                   \fR# Decorate with both arrows\fP
+                 invis                 \fR# Make primitive invisible\fP
+                 solid                 \fR# Make closed figure solid\fP
+                 fill <expr>           \fR# Set fill density for figure\fP
+                 same                  \fR# Copy size of previous object\fP
+                 <text> <text> ...     \fR# Text within object\fP
+                 <expr>                \fR# Motion in the current direction\fR
+.R
+.DE
+.LP
+Missing attributes are supplied from defaults; inappropriate ones are
+silently ignored.  For lines, splines, and arcs, height and width
+refer to arrowhead size.
+.PP
+The \f(CWat\fP primitive sets the center of the current object.  The
+\f(CWwith\fP attribute fixes the specified feature of the given object
+to a specified location.
+.PP
+The \f(CWsolid\fR primitive is not yet supported in GNU \fBgpic\fR.
+.PP
+The \f(CWby\fR primitive is not documented in the tutorial portion of
+the Kernighan paper, and should probably be considered unreliable.
+.PP
+The primitive \fBarrow\fR is a synonym for \fBline ->\fR.
+.DS
+.CW
+<text>  ::=      <string> [ <placement> ... ]
+                 sprintf("format", <expr> ...) [ <placement> ... ]
+
+<placement> ::=  center | ljust | rjust | above | below
+.R
+.DE
+.LP
+Text is normally an attribute of some object, in which case successive
+strings are vertically stacked and centered on the object's center by
+default.  Standalone text is treated as though placed in an invisible
+box.
+.PP
+A text item consists of a string or sprintf-expression, optionally
+followed by positioning information.  Text or format strings may
+contain {gtn}roff font changes, size changes, and local motions,
+provided those changes are undone before the end of the current item.
+.PP
+A position is an (x, y) coordinate pair.  There are lots of different
+ways to specify positions:
+.DS
+.CW
+<position>  ::=  <expr> , <expr>
+                 <place> {+-} <expr> , <expr> 
+                 <place> {+-} ( <expr> , <expr> )
+                 ( <position> , <position> )
+                 <expr> [of the way] between <position> and <position> 
+                 <expr> '<' <position> , <position> '>'
+                 ( <position> )
+
+<place>  ::=     <label> [ <dot-corner> ]
+                 <corner> of <label>
+                 [0|1|2|3|4|5|6|7|8|9]th [last] <primitive> <dot-corner>
+                 <expr>'th [last]<primitive> <dot-corner>
+                 <corner> of [0|1|2|3|4|5|6|7|8|9]th [last] <primitive>
+                 <corner> of <expr>'th [last] <primitive>
+                 Here
+
+<dot-corner> ::= .n | .e | .w | .s | .ne | .nw | .se | .sw | .c | .start | .end
+
+<corner> ::=     top | bot | left | right | start | end
+.R
+.DE
+.LP
+As Kernighan notes, "since barbarisms like \fB1th\fP and \fB3th\fP are
+barbaric, synonyms like \fB1st\fP and \fB3rd\fP are accepted as well".
+Objects of a given type are numbered from 1 upwards in order of
+declaration; the \fBlast\fP modifier counts backwards.
+.PP
+The "'th" form (which allows you to select a previous object with an
+expression, as opposed to a numeric literal) is bnot documented in DWB
+\fBpic\fR(1).
+.PP
+The following style variables control output:
+.TS H
+tab(@), linesize(2);
+lb | lb | lb
+l | n | l.
+.sp 2p
+Style Variable@Default@What It Does
+.sp 2p
+_
+.sp 2p
+.TH
+boxht@0.5@Default height of a box
+boxwid@0.75@Default height of a box
+lineht@0.5@Default length of vertical line
+linewid@0.75@Default length of horizontal line
+arcrad @0.25@Default radius of an arc
+circlerad@0.25@Default radius of a circle
+ellipseht@0.5@Default height of an ellipse
+ellipsewid@0.75@Default width of an ellipse
+moveht@0.5@Default length of vertical move
+movewid@0.75@Default length of horizontal move
+textht@0@Default height of box enclosing a text object
+textwid@0@Default width of box enclosing a text object
+arrowht@0.1@Length of arrowhead along shaft
+arrowwid@0.05@Width of rear of arrowhead
+arrowhead@1@Enable/disable arrowhead filling
+dashwid@0.05@Interval for dashed lines
+maxpswid@11@Maximum width of picture
+maxpsht@8.5@Maximum height of picture
+scale@1@Unit scale factor
+fillval@0.5@Default fill value
+.sp 5p
+_
+.TE
+Any of these can be set by assignment, or reset using the \fBreset\fP
+statement.  Style variables assigned within [] blocks are restored to
+their beginning-of-block value on exit; top-level assignments persist
+across pictures.  Dimensions are divided by \fBscale\fR on output.
+.PP
+All \fBpic\fP expressions are evaluated in floating point; units
+default to inches.  Expressions have the following simple grammar,
+with semantics very similar to C expressions:
+.DS
+.CW
+<expr> ::=       <expr> <op> <expr>
+                 ! <expr>
+                 ( <expr> )
+                 - <expr>
+                 <variable>
+                 <number>
+                 <place> .x
+                 <place> .y
+                 <place> .ht
+                 <place> .wid
+                 <place> .rad
+                 sin(<expr>)
+                 cos(<expr>)
+                 log(<expr>)
+                 exp(<expr>)
+                 sqrt(<expr>)
+                 max(<expr>, <expr>...)
+                 atan2(<expr>, <expr>)
+                 min(<expr>, <expr>...)
+                 int(<expr>)
+                 rand()
+
+<op>  :=         + | - | * | / | % | ^ |
+                 != | == | '<' | '>' | >= | <= |
+                 '||' | &&
+.R
+.DE
+.LP
+Both \fBexp\fP and \fBlog\fP are base 10; \fBint\fP does integer
+truncation; and \fBrand()\fP returns a random number in [0-1).
+.PP
+There are \fBdefine\fP and \fBundef\fR statements which are not part
+of the grammar (they behave as pre-processor macros to the language).
+These may be used to define pseudo-functions.
+.DS
+.CW
+\fBdefine\fP \fIname\fP \fB{\fP \fIreplacement text \fB}\fP
+.R
+.DE
+.LP
+This defines \fIname\fR as a macro to be replaced by the replacement
+text (not including the braces).  The macro may be called as 
+.DS
+.CW
+\fIname\fB(\fIarg1, arg2, ... argn\fB)\fR
+.R
+.DE
+.LP
+The arguments (if any) will be substituted for tokens $1, $2 ... $n
+appearing in the replacement text. To undefine a mcro, say \fBundef\fP
+\fIname\fR, specifying the name to be undefined.
+.\"%%POSTLUDE%%
+.NH 1
+History and Acknowledgements
+.PP
+Original \fBpic\fP was written to go with Joseph Ossanna's original
+\&\fItroff\fP(1) by Brian Kernighan, and later re-written by Kernighan
+with substantial enhancements (apparently as part of the evolution of
+\&\fItroff\fP(1) into \&\fIditroff\fP(1) to generate
+device-independent output).
+.PP
+The language had been inspired by some earlier graphics languages
+including \fBideal\fP and \fBgrap\fP.  Kernighan credits Chris van Wyk
+(the designer of \fBideal\fP) with many of the ideas that went into
+\fBpic\fP.
+.EQ
+delim $$
+.EN
+.PP
+The \fBpic\fP language was originally described by Brian Kernighan in
+Bell Labs Computing Science Technical Report #116 (you can obtain a
+PostScript copy of the revised version, [1], by sending a mail message to
+\&\fInetlib@research.att.com\fP with a body of \&`send 116 from
+research/cstr'.).  There have been two revisions, in 1984 and 1991.
+.PP
+The document you are reading effectively subsumes Kernighan's
+description; it was written to fill in lacunae in the exposition and
+integrate in descriptions of the GNU \fIgpic\fP(1) features.
+.PP
+The GNU \fBgpic\fR implementation was written and is maintained by
+James Clark \fI<jjc@jclark.com>\fP.
+.NH 1
+Bibliography
+.IP 1.
+Kernighan, B. W. \fBPIC -- A Graphics Language for Typesetting
+(Revised User Manual)\fP Bell Labs Computing Science Technical Report
+#116, December 1991.
+.IP 2.
+Van Wyk, C.J. \fBA high-level language for specifying pictures\fP \fIACM
+Transactions On Graphics\fP 1,2 (1982) 163-182.