Put in minimal simpset to avoid excessive simplification,
just as in revision 1.9 of HOL/indrule.ML
(* Title: Substitutions/setplus.thy
Author: Martin Coen, Cambridge University Computer Laboratory
Copyright 1993 University of Cambridge
Minor additions to HOL's set theory
*)
Setplus = Set +
rules
ssubset_def "A < B == A <= B & ~ A=B"
end