Declaration of type 'nat' as a datatype (this allows usage of
exhaust_tac and induct_tac on type 'nat'). Moved some proofs
using natE from NatDef.ML to Nat.ML.
Isabelle BUGS -- history of reported faults
===========================================
1. Symptom: hyp_subst_tac does nothing if the selected equality involves type
unknowns. Cause: it uses the simplifier, which ignores such equalities.
Fix: check for type unknowns in hypsubst/inspect_pair; change interface
function dest_eq to return the type of the equality. (lcp, 5/11/97)
2. Symptom: read_instantiate_sg has problems instantiating types in some
simultaneous instantiations (Message-id: <199710301432.PAA20594@sirius.Informatik.Uni-Bremen.DE> on isabelle-users)