proposition: Lemma unitb_umfilt p f : unitb f -> unitb (um_filter p f).
proof: Proof. by move/unitbE->; rewrite umfilt0 unitb0. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)