Single Pair of Parenthesis Around Proof Required

To be compliant with SMT-LIB it was suggested to change the proof output such that it is surrounded by a pair of parenthesis.

  unsat
       (
       (assume a0 (! (not (=> (! (= (of_nat$ (nat$ (x_000$ 3))) 5) :named @p_1) @p_1)) :named @p_2))
       (step t0 (cl @p_1) :rule not_implies1 :premises (a0))
       (step t1 (cl (not @p_1)) :rule not_implies2 :premises (a0))
       (step t2 (cl) :rule resolution :premises (t0 t1))
       )

This requires small changes to the grammar and adapting the examples of proofs in the standard.