proposition: Lemma ujoinA x y z : ujoin x (ujoin y z) = ujoin (ujoin x y) z.
proof: Proof. by congr mk3; rewrite joinA. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq bigop fintype finset) pcm(options axioms prelude seqperm pred seqext)