src/HOL/SMT/Examples/cert/z3_nat_arith_06

fix soundness bug in Nitpick's "destroy_constrs" optimization

(benchmark Isabelle
:extrasorts ( T1)
:extrafuns (
  (uf_1 Int T1)
  (uf_2 T1 Int)
  (uf_3 Int)
 )
:assumption (forall (?x1 T1) (= (uf_1 (uf_2 ?x1)) ?x1))
:assumption (forall (?x2 Int) (implies (<= 0 ?x2) (= (uf_2 (uf_1 ?x2)) ?x2)))
:assumption (forall (?x3 Int) (implies (< ?x3 0) (= (uf_2 (uf_1 ?x3)) 0)))
:assumption (not (= (uf_2 (uf_1 (ite (< uf_3 0) (~ uf_3) uf_3))) (ite (< uf_3 0) (~ uf_3) uf_3)))
:formula true
)