proposition: Lemma subset_iterS i : iterF i \subset iterF i.+1.
proof: Proof. by elim: i => [| i IHi]; rewrite /= ?sub0set ?F_mono. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)