$ z3 unknown.smt2
unknown
$ z3 tactic.default_tactic=smt sat.euf=true unknown.smt2
(declare-fun str.prefixof (String String) Bool) not handled
(declare-fun str.from_int (Int) String) not handled
unknown
$ z3-4.8.10 unknown.smt2
sat
$ cat unknown.smt2
(declare-const x Int)
(declare-const x9 Bool)
(assert (or x9 (str.prefixof (str.from_int x) (str.from_int (- x)))))
(check-sat)
git-bisect: 377d060x9 = true.