defcustom :type to use for a key sequence?

["Drew Adams" <drew.adams@oracle.com>]

I sent this question to gnu-emacs-help a while back. I got one reply saying
that there is no better choice, for now, than `sexp', and that this
(key-sequence typing) was to be reviewed for possible enhancement after the
release.

Can someone confirm that `sexp' is the best choice here, for now? Thanks.

        Suppose I have this:

        (defcustom the-key [(meta ?\ )] "...")
        (define-key my-map the-key 'the-cmd))

        A user might customize which key sequence is bound to `the-cmd'.

        What :type is appropriate for the defcustom? I don't see a type
        ready-made for key sequences.  Just `string'?  Or is a `choice' of
        `string' and (what kind of?) `vector' appropriate?  How can the
        :type allow for any key sequence (besides using just `sexp')?  Thx.