Put in minimal simpset to avoid excessive simplification,
just as in revision 1.9 of HOL/indrule.ML
(* Title: HOL/AxClasses/Tutorial/Xor.thy
ID: $Id$
Author: Markus Wenzel, TU Muenchen
Define overloaded constants "<*>", "inv", "1" on type "bool".
*)
Xor = Group +
defs
prod_bool_def "x <*> y == x ~= (y::bool)"
inv_bool_def "inv x == x::bool"
unit_bool_def "1 == False"
end