recursive typing question

I'm trying to wrap a certain type in the R internals headers, and the
type is defined recursively, and I'm not sure how to get at it.

In particular, I'm trying to wrap a type called SEXPREC.  A SEXPREC
consists of some header information, including some SEXPREC *'s, which
I can handle using :pointer-self.  The SEXPREC also contains a union
of a bunch of different types, each of which contain some SEXPREC *'s,
and it's these I'm not sure how to get at --- I can't define them
first, because they have SEXPREC's inside them, but I can't define
SEXPREC first either.  My current idea is just to rely on the fact
that in practice, SEXPREC *'s and void *'s will be the same size, so
declare the subtypes as containing (void *)'s.  Any thoughts?

rif