bug#50928: remove-dups

[Tak Kunihiro <tkk@misasa.okayama-u.ac.jp>]

>> Now I can find its existence in (info
>> "(elisp) Sequence Functions").  I wonder how I could have reached to
>> the function by myself.
>>
>> How did you find it? (apropos-documentation "duplicate")?
> 
> I just...  knew about seq.el.  The cross-referencing between the older
> sequence functions and seq.el is rather lacking -- basically all these
> older functions should probably reference something in seq.el in their
> doc strings.

How about something like below?

commit xxx
Author: yyy
Date:   zzz

    Add references to a newer function `seq-uniq' in seq.el
    
    * lisp/subr.el (delete-dups):
    * doc/lispref/lists.texi (Sets And Lists):
    * doc/lispref/lists.texi (delete-dups):  Refer to `seq-uniq' (bug#50928).

diff --git a/lisp/subr.el b/lisp/subr.el
index e4819c4b2b..228d2e0c22 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -696,7 +696,7 @@ delete-dups
   "Destructively remove `equal' duplicates from LIST.
 Store the result in LIST and return it.  LIST must be a proper list.
 Of several `equal' occurrences of an element in LIST, the first
-one is kept."
+one is kept.  See `seq-uniq' for non-destructive operation."
   (let ((l (length list)))
     (if (> l 100)
         (let ((hash (make-hash-table :test #'equal :size l))

diff --git a/doc/lispref/lists.texi b/doc/lispref/lists.texi
index 75641256b6..66c556ecd0 100644
--- a/doc/lispref/lists.texi
+++ b/doc/lispref/lists.texi
@@ -1227,13 +1227,13 @@ Sets And Lists
 @cindex lists as sets
 @cindex sets
 
-  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).  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}.
+  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).  You can remove
+@code{equal} duplicates using @code{delete-dups} or @code{seq-uniq}.
+Other useful functions for sets include @code{memq} and @code{delq},
+and their @code{equal} versions, @code{member} and @code{delete}.
 
 @cindex CL note---lack @code{union}, @code{intersection}
 @quotation
@@ -1489,7 +1489,8 @@ Sets And Lists
 This function destructively removes all @code{equal} duplicates from
 @var{list}, stores the result in @var{list} and returns it.  Of
 several @code{equal} occurrences of an element in @var{list},
-@code{delete-dups} keeps the first one.
+@code{delete-dups} keeps the first one.  See @code{seq-uniq} for
+non-destructive operation.
 @end defun
 
   See also the function @code{add-to-list}, in @ref{List Variables},