[eqn] Add `delim on'.

* src/preproc/eqn/lex.cpp (start_delim_saved, end_delim_saved): New
global variables.
(do_delim): Handle `delim on' to restore previous start and end
delimiters.

* src/preproc/eqn/eqn.man, NEWS: Document it.

2 files changed, 31 insertions, 3 deletions

diff --git a/src/preproc/eqn/eqn.man b/src/preproc/eqn/eqn.man
index f4c6457f..203d9033 100644
--- a/src/preproc/eqn/eqn.man
+++ b/src/preproc/eqn/eqn.man
@@ -1,5 +1,5 @@
 .ig
-Copyright (C) 1989-2000, 2001, 2004, 2005, 2007, 2009
+Copyright (C) 1989-2001, 2004, 2005, 2007, 2009, 2013
   Free Software Foundation, Inc.
 
 Permission is granted to make and distribute verbatim copies of
@@ -131,6 +131,9 @@ Recognize
 and
 .B .EN
 even when followed by a character other than space or newline.
+Also, the statement
+.RB ` "delim on" '
+is not handled specially.
 .TP
 .B \-N
 Don't allow newlines within delimiters.
@@ -243,6 +246,23 @@ There are some references to the differences between \*(tx and GNU
 below;
 these may safely be ignored if you do not know \*(tx.
 .
+.SS Controlling delimiters
+.
+If not in compatibility mode,
+.B eqn
+recognizes
+.
+.RS
+.LP
+.B delim on
+.RE
+.
+.LP
+to restore the delimiters which have been previously disabled
+with a call to
+.RB ` "delim off" '.
+If delimiters haven't been specified, the call has no effect.
+.
 .SS Automatic spacing
 .B eqn
 gives each component of an equation a type, and adjusts the spacing
diff --git a/src/preproc/eqn/lex.cpp b/src/preproc/eqn/lex.cpp
index b328801d..899ecaeb 100644
--- a/src/preproc/eqn/lex.cpp
+++ b/src/preproc/eqn/lex.cpp
@@ -1,6 +1,5 @@
 // -*- C++ -*-
-/* Copyright (C) 1989, 1990, 1991, 1992, 2000, 2001, 2002, 2003, 2005, 2007,
-                 2008, 2009
+/* Copyright (C) 1989-1992, 2000-2003, 2005, 2007-2009, 2013
    Free Software Foundation, Inc.
      Written by James Clark (jjc@jclark.com)
 
@@ -1058,6 +1057,9 @@ void do_ifdef()
   }
 }
 
+char start_delim_saved = '\0';
+char end_delim_saved = '\0';
+
 void do_delim()
 {
   int c = get_char();
@@ -1069,8 +1071,14 @@ void do_delim()
   else {
     if (c == 'o' && d == 'f' && peek_char() == 'f') {
       (void)get_char();
+      start_delim_saved = start_delim;
+      end_delim_saved = end_delim;
       start_delim = end_delim = '\0';
     }
+    else if (c == 'o' && d == 'n' && !compatible_flag) {
+      start_delim = start_delim_saved;
+      end_delim = end_delim_saved;
+    }
     else {
       start_delim = c;
       end_delim = d;