Index: doc/groff.texinfo
===================================================================
RCS file: /sources/groff/groff/doc/groff.texinfo,v
retrieving revision 1.291
diff -u -r1.291 groff.texinfo
--- doc/groff.texinfo	2 Jun 2010 14:26:25 -0000	1.291
+++ doc/groff.texinfo	28 Jul 2010 15:37:33 -0000
@@ -5128,6 +5128,32 @@
 `true' because @code{gtroff} treats both negative numbers and zero as
 `false'.}  See below for the use of unary operators in motion requests.
 
+@cindex logical not, limitation in expression
+@cindex expression, limitation of logical not in
+The logical not operator,
+as described above,
+works only within @code{if} and @code{while} requests.
+Furthermore, it may appear
+only at the beginning of an expression,
+and negates the entire expression.
+Attempting to insert the @samp{!} operator
+within the expression results in a
+@samp{numeric expression expected} warning.  This
+maintains bug-compatibility
+with old versions of @code{troff}.
+
+Example:
+
+@Example
+.nr X 1
+.nr Y 0
+.\" This does not work as expected
+.if (\n[X])&(!\n[Y]) .nop X only
+.
+.\" Use this construct instead
+.if (\n[X]=1)&(\n[Y]=0) .nop X only
+@endExample
+
 @item
 @cindex extremum operators (@code{>?}, @code{<?})
 @cindex operators, extremum (@code{>?}, @code{<?})
Index: man/groff.man
===================================================================
RCS file: /sources/groff/groff/man/groff.man,v
retrieving revision 1.104
diff -u -r1.104 groff.man
--- man/groff.man	24 Apr 2009 20:43:20 -0000	1.104
+++ man/groff.man	28 Jul 2010 15:37:35 -0000
@@ -884,7 +884,7 @@
 .BI ! N
 True if the value of
 .I I
-is\~0.
+is\~0 (see below).
 .
 .TPx
 .BI \[aq] s1 \[aq] s2 \[aq]
@@ -898,7 +898,8 @@
 True if string\~\c
 .I s1
 is not identical to string\~\c
-.IR s2 .
+.I s2
+(see below).
 .
 .TPx
 .BI c ch
@@ -952,6 +953,13 @@
 .
 .RE
 .PD
+.P
+Note that the
+.B !
+operator may only appear at the beginning of an expression,
+and negates the entire expression.
+This maintains bug-compatibility with AT&T
+.IR troff .
 .
 .
 .\" --------------------------------------------------------------------