bug#26540: 25.2; [PATCH] Add cl-set-equal to test for set equality

[Drew Adams]

> This patch adds cl-seq-equal to test whether two lists have the
> same elements. I.e., if every element of LIST1 also appears in
> LIST2 and if every element of LIST2 also appears in LIST1.

Common Lisp (and the Emacs emulation) already has set
functions that do this - `[cl-]set-exclusive-or', for
example.

https://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node152.html