proposition: Lemma iter_sub_fix k : iterF k \subset fixset.
proof: Proof. have [/subset_iter //|/ltnW/subnK<-] := leqP k n; by rewrite /iterF iterD fixsetKn. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)