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 (if_then_else (forall (?x1 Int) (or (< ?x1 0) (< 0 ?x1))) false true))
:formula true
)