proposition: Lemma uundef_join6 x : uundef \+ x = uundef.
proof: Proof. by rewrite pcmE /= !undef_join. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq bigop fintype finset) pcm(options axioms prelude seqperm pred seqext)