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: LatPreInsts.thy
ID: $Id$
Author: Markus Wenzel, TU Muenchen
*)
LatPreInsts = OrdInsts + Lattice + CLattice