proposition: Lemma fix_order_eq0 (x : T) : (fix_order x == 0) = (x \notin fixset).
proof: Proof. by rewrite -fix_order_gt0 -ltnNge ltnS leqn0. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)