Now Datatype.occs_in_prems prints the necessary warning ITSELF.
It is also easier to invoke and even works if the induction variable
is a parameter (rather than a free variable).
(* Title: Option.thy
ID: $Id$
Author: Tobias Nipkow
Copyright 1994 TU Muenchen
Datatype 'a option
*)
Option = Arith +
datatype 'a option = None | Some 'a
end