proposition: Lemma in_iter (x : T) k : x \in fixset -> fix_order x <= k -> x \in iterF k.
proof: Proof. by move=> x_in xk; rewrite in_iter_fixE fix_order_gt0 x_in xk. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)