proposition: Lemma ujoinC_fin x y : ujoin x y = ujoin y x.
proof: Proof. by apply: fin_ext=>a; rewrite !sel_fin joinC. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq bigop fintype finset) pcm(options axioms prelude seqperm pred seqext)