proposition: Lemma umall2_undef : um_all2 undef undef.
proof: Proof. by move=>k; rewrite /optionR /= find_undef. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)