proposition: Lemma codomf_rem f (A : {fset K}) : codomf f.[\ A] = [fset f k | k : domf f & val k \notin A].
proof: Proof. rewrite codomf_restrict. by apply: eq_imfset => //= x /=; rewrite -!topredE /= !inE (valP x) andbT. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)