added proof reconstructon for Z3,
added certificates for simpler re-checking of proofs (no need to invoke external solvers),
added examples and certificates for all examples,
removed Unsynchronized.ref (in smt_normalize.ML)
(benchmark Isabelle
:assumption (not (exists (?x1 Int) (forall (?x2 Int) (?x3 Int) (implies (and (< 0 ?x2) (< 0 ?x3)) (< 0 (+ ?x2 ?x3))))))
:formula true
)