proposition: Lemma umfoldr_undef a z0 d : um_foldr a z0 d undef = d.
proof: Proof. by rewrite /um_foldr valid_undef. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)