proposition: Lemma umcnt_umfilt p q f : um_count p (um_filter q f) = um_count (predI p q) f.
proof: Proof. by rewrite /um_count umfilt_predI. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)