proposition: Lemma in_fset_cat (xs ys : seq K) a : (a \in [fset[key] x in xs ++ ys]) = ((a \in [fset[key] x in xs]) || (a \in [fset[key] x in ys])).
proof: Proof. by rewrite !mem_imfset//= mem_cat. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)