Printing elisp char literals

["Stefan Monnier" <monnier+gnu/emacs@rum.cs.yale.edu>]

Let's say I have an integer.  How can I check whether it's a char ?
`char-valid-p' is only part of the answer, because I'd like to know
if the integer is a char-with-modifiers also.  The best I could come
up with right now is:

   (char-valid-p (logand char ?\x3fffff))

Is the mask available somewhere or do I simply have to hardcode it
like that ?

Furthermore, in order to print this integer in a form like ?\C-a,
what should I do ?  The best I could come up with is:

    (let ((s (single-key-description char)))
      (while (string-match "\\`\\(\\\\[MCAHSs]-\\)*\\(\\)[MCAHSs]-." s)
        (setq s (replace-match "\\" t t s 2)))
      (concat "?" s))


-- Stefan