defcustom :type to use for a key sequence?

[Drew Adams]

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.