proposition: Lemma BRopp_fb: bijection (Lf BRopp BR BR).
proof: Proof. apply: lf_bijective. - by move => t /RSo tz. - apply: BRopp_inj. - move => y yz; exists (BRopp y); first by apply: RSo. by symmetry;apply BRopp_K. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)