proposition: Lemma leq_imfset_card (T T' : choiceType) (f : T -> T') (p : finmempred _) : (#|` imfset key f p| <= size (enum_finmem p))%N.
proof: Proof. by rewrite unlock size_seq_fset (leq_trans (size_undup _)) ?size_map. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)