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