added Pair_eq to pair_ss in prod.ML
removed it locally in llist.ML because preconditions of the form <a,b> =
<?x,?y>, which used to be solved by reflexivity, now rewrote to a = ?x & b =
?y, which is not solved by reflexivity.
(* 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