proposition: Lemma in_fset (p : finmempred K) (k : K) : (k \in imfset key id p) = in_mem k p.
proof: Proof. by rewrite mem_imfset; apply: inj_id. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)