bug#50110: 26.3; Doc of `assoc': TESTFN optional arg

[Drew Adams]

Neither the `assoc' doc string nor the Elisp manual, node `Association
Lists' explains the order of the args that get passed to TESTFN.

It's just as reasonable to expect that the pattern is the first arg to
TESTFN, instead of, as is actually the case, the second arg.

E.g., one could suppose that this would work (return the first element):

(setq trees '(("pine" . "cones") ("oak" . "acorns")))

(assoc ".*ine" trees #'string-match-p)

But in fact this is what you need:

(assoc ".*ine" trees (lambda (x regexp) (string-match-p regexp x)))

Please consider documenting the arg order for TESTFN.

In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.19042
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''