proposition: Lemma find_umfiltk k (p : pred K) f : find k (um_filterk p f) = if p k then find k f else None.
proof: Proof. by rewrite find_umfilt /=; case: (find _ _)=>[a|]; case: ifP. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)