proposition: Corollary slice_has p s i : has p (&:s i) = has (fun j => j \in i) (findall p s).
proof: Proof. by rewrite !has_count slice_count. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path interval order) pcm(options prelude ordtype seqext)