proposition: Lemma find_undef k : find k (undef : U) = None.
proof: Proof. by rewrite /undef /= !umEX /UM.find. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)