(Sets And Lists): Add delete-dups.

1 files changed, 16 insertions, 1 deletions

diff --git a/lispref/lists.texi b/lispref/lists.texi
index b123de5ab10..cb033118984 100644
--- a/lispref/lists.texi
+++ b/lispref/lists.texi
@@ -1223,7 +1223,8 @@ useful example of @code{sort}.
   A list can represent an unordered mathematical set---simply consider a
 value an element of a set if it appears in the list, and ignore the
 order of the list.  To form the union of two sets, use @code{append} (as
-long as you don't mind having duplicate elements).  Other useful
+long as you don't mind having duplicate elements).  You can remove
+@code{equal} duplicates using @code{delete-dups}.  Other useful
 functions for sets include @code{memq} and @code{delq}, and their
 @code{equal} versions, @code{member} and @code{delete}.
 
@@ -1433,6 +1434,20 @@ equal, and unibyte strings are converted to multibyte prior to
 comparison.
 @end defun
 
+@defun delete-dups list
+This function destructively removes all @code{equal} duplicates from
+@var{list} and returns the result.  Of several @code{equal}
+occurrences of an element in @var{list}, @code{delete-dups} keeps the
+last one.
+
+The value of @var{list} after a call to this function is undefined.
+Usually, we store the return value back in @var{list}:
+
+@example
+(setq list (delete-dups list))
+@end example
+@end defun
+
   See also the function @code{add-to-list}, in @ref{Setting Variables},
 for another way to add an element to a list stored in a variable.