diff --git a/doc/ref/api-compound.texi b/doc/ref/api-compound.texi
index b291071cb..b69a3a25f 100644
--- a/doc/ref/api-compound.texi
+++ b/doc/ref/api-compound.texi
@@ -1,6 +1,6 @@
 @c -*-texinfo-*-
 @c This is part of the GNU Guile Reference Manual.
-@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004
+@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2005
 @c   Free Software Foundation, Inc.
 @c See the file guile.texi for copying conditions.
 
@@ -553,7 +553,9 @@ predicate @var{pred}.  The elements in the result list have the same
 order as in @var{lst}.  The order in which @var{pred} is applied to
 the list elements is not specified.
 
-@code{filter!} is allowed, but not required to modify the structure of
+@code{filter} does not change @var{lst}, but the result may share a
+tail with it.  @code{filter!} may modify @var{lst} to construct its
+return.
 @end deffn
 
 @node List Searching