proposition: Lemma pos_joinC x y : pos_join x y = pos_join y x.
proof: Proof. by apply/eqP; rewrite /pos_join -val_eqE /= maxnC. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq bigop fintype finset) pcm(options axioms prelude seqperm pred seqext)