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 (forall (?x1 Int) (?x2 Int) (or (< 2 (+ ?x1 ?x2)) (or (= (+ ?x1 ?x2) 2) (< (+ ?x1 ?x2) 2)))))
:formula true
)