(set-logic QF_UF)
(declare-sort S 0)
(declare-fun f (S) S)
(declare-fun x1 () S)
(declare-fun x2 () S)
(declare-fun x3 () S)
(declare-fun x4 () S)
(declare-fun x5 () S)
(declare-fun fx1 () S)
(declare-fun ffx1 () S)

(assert (= x1 (f x1)))
(assert (= x2 (f x2)))
(assert (= fx1 (f x1)))
(assert (= ffx1 (f fx1)))
(assert (not (= x1 ffx1)))
(check-sat)