proposition: Lemma prod_undef_join x : (@undef U, @undef V) \+ x = (@undef U, @undef V).
proof: Proof. by rewrite [x]prod_eta /= pcmPJ !undef_join. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq bigop fintype finset) pcm(options axioms prelude seqperm pred seqext)