cl-reduce doc fix

* lisp/emacs-lisp/cl-seq.el (cl-reduce): Clarify what happens when
SEQ is the empty list (bug#37400).

1 files changed, 5 insertions, 0 deletions

diff --git a/lisp/emacs-lisp/cl-seq.el b/lisp/emacs-lisp/cl-seq.el
index a15c994bc1a..520d450481f 100644
--- a/lisp/emacs-lisp/cl-seq.el
+++ b/lisp/emacs-lisp/cl-seq.el
@@ -133,6 +133,11 @@ If :INITIAL-VALUE is specified, it is added to the front of SEQ.
 If SEQ is empty, return :INITIAL-VALUE and FUNCTION is not
 called.
 
+If SEQ is empty and no :INITIAL-VALUE is given, then the function
+is called with zero arguments, and reduce returns whatever
+function does. This is the only case where the function is called
+with other than two arguments.
+
 \n(fn FUNCTION SEQ [KEYWORD VALUE]...)"
   (cl--parsing-keywords (:from-end (:start 0) :end :initial-value :key) ()
     (or (listp cl-seq) (setq cl-seq (append cl-seq nil)))