proposition: Lemma BZopp_fb: bijection (Lf BZopp BZ BZ).
proof: Proof. apply: lf_bijective. - by move => t /ZSo. - apply: BZopp_inj. - by move => y yz; exists (BZopp y); [apply:ZSo | rewrite (BZopp_K yz)]. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)