Problem with Recusive Type

[DavePearce]

(see also #55)
The following fails for reasons unknown:

type nat is (int n) where n >= 0
type pos is (nat p) where p > 0

type expr is nat | {expr lhs}
type posExpr is pos | {posExpr lhs}

function f(posExpr e1) -> expr:
    return e1