How to declare a recursive customization type?

[David Kastrup <David.Kastrup@t-online.de>]

Hello, I have a certain recursive data structure which I want to
define as a custom type.  Currently I use the following contraption

(defconst preview-expandable-string
  ((lambda (f) (funcall f (funcall f 'sexp)))
   (lambda (x)
     `(choice
       string
       (cons :tag "List with separators"
	(string :tag "Separator")
	     (choice (symbol :tag "Variable to concatenate.")
		     (repeat :tag "Explicit elements" ,x)))
       (symbol :tag "Indirect variable"))))
  "Type to be used for `preview-string-expand'.
Recursive definitions are not popular with Emacs,
so we define this type just two levels deep.  This
kind of strings can either be just a string, or a
cons cell with a separator string in the CAR, and either
an explicit list of elements in the CDR, or a symbol to
be consulted recursively.")

which is going to be used just as
:type preview-expandable-string
in a defcustom.  However, this sort of broken-off recursion is not
satisfactory.

One should probably define preview-expandable-string as a proper type
to be used as
:type 'preview-expandable-string
in a defcuston, and then it will probably be able to refer to itself
cleanly.

But how does one achieve that?  I digged through what I could find
about both customize, widget, and wid-edit, and have again to admit
that I am too stupid to find how to do what must be a simple thing,
really.

-- 
David Kastrup, Kriemhildstr. 15, 44793 Bochum