Add conditional operator xor to subr.el

Suggested by Oleh Krehel and implemented by Basil Contovounesios in
the following thread:
https://lists.gnu.org/archive/html/emacs-devel/2019-07/msg00547.html

* lisp/array.el (xor): Move unused function from here...
* lisp/subr.el: ...to here, and improve.
* lisp/gnus/spam.el (spam-xor):
* lisp/play/5x5.el (5x5-xor):
* lisp/proced.el (proced-xor):
* lisp/progmodes/idlwave.el (idlwave-xor):
* lisp/vc/diff-mode.el (diff-xor): Define as obsolete aliases of,
and replace all uses with, xor.
* lisp/jsonrpc.el: Remove unused dependency on array.el.
* lisp/org/org.el (org-xor): Move from here...
* lisp/org/org-compat.el (org-xor): ...to here, as a compatibility
shim for xor.
* lisp/progmodes/idlw-shell.el (idlwave-shell-enable-all-bp):
* lisp/simple.el (exchange-point-and-mark):
* lisp/windmove.el (windmove-display-in-direction): Use xor.
* lisp/strokes.el (strokes-xor): Remove commented-out xor
implementation.

* doc/lispref/control.texi (Control Structures): Extend menu entry
for new combining condition.
(Combining Conditions):
* etc/NEWS (Lisp Changes): Document xor.

* test/lisp/subr-tests.el (subr-test-xor): New test.

1 files changed, 0 insertions, 6 deletions

diff --git a/lisp/strokes.el b/lisp/strokes.el
index 0c671c43ac2..6edf58c7b6b 100644
--- a/lisp/strokes.el
+++ b/lisp/strokes.el
@@ -1524,12 +1524,6 @@ Encode/decode your strokes with \\[strokes-encode-buffer],
   (or (eq char ?\s)
       (eq char ?*)))
 
-;;(defsubst strokes-xor (a b)  ### Should I make this an inline function? ###
-;;  "T if one and only one of A and B is non-nil; otherwise, returns nil.
-;;NOTE: Don't use this as a numeric xor since it treats all non-nil
-;;      values as t including `0' (zero)."
-;;  (eq (null a) (not (null b))))
-
 (defsubst strokes-xpm-encode-length-as-string (length)
   "Given some LENGTH in [0,62) do a fast lookup of its encoding."
   (aref strokes-base64-chars length))