Implement a fallback character request `.fchar'.

* src/roff/troff/charinfo.h (charinfo): New flag `fallback'.
(is_fallback): New inline function.
* src/roff/troff/input.cc (do_define_character): New function.
(define_character): Call `do_define_character'.
(define_fallback_character): New function.
(init_input_requests): Add `fchar'.
(charinfo::charinfo): Updated.
(charinfo::set_macro): New argument to set `fallback' flag.
* src/roff/troff/node.cc (make_glyph_node, make_node,
node::add_char): Check `fallback' flag.
* NEWS, man/groff_diff.man, man/groff_man: Updated.

2 files changed, 24 insertions, 1 deletions

diff --git a/man/groff.man b/man/groff.man
index a64e9f0e..b845985a 100644
--- a/man/groff.man
+++ b/man/groff.man
@@ -987,7 +987,7 @@ to
 .REQ .char c anything
 Define character
 .argument c
-to string
+as string
 .argument anything .
 .
 .REQ .chop object
@@ -1171,12 +1171,19 @@ Disable field mechanism.
 Set field delimiter to
 .argument a
 and pad character to space.
+.
 .REQ .fc a b
 Set field delimiter to
 .argument a
 and pad character to
 .argument b .
 .
+.REQ .fchar c anything
+Define fallback character
+.argument c
+as string
+.argument anything .
+.
 .REQ .fi
 Fill output lines.
 .
diff --git a/man/groff_diff.man b/man/groff_diff.man
index fe3ae95b..644639e1 100644
--- a/man/groff_diff.man
+++ b/man/groff_diff.man
@@ -1088,6 +1088,22 @@ See the description of the
 request for more information on font families.
 .
 .TP
+.BI .fchar\  c\ string
+Define fallback character
+.I c
+to be
+.IR string .
+The syntax of this request is the same as the
+.B char
+request; the only difference is that a character defined with
+.B char
+hides the glyph with the same name in the current font, whereas a character
+defined with
+.B fchar
+is checked only if the particular glyph isn't found in the current font.
+This test happens before checking special fonts.
+.
+.TP
 .BI .fspecial\  f\ s1\ s2\|.\|.\|.
 When the current font is
 .IR f ,