proposition: Lemma ffix_order_eq0 (x : T) : (ffix_order x == 0) = (x \notin fixfset).
proof: Proof. by rewrite -ffix_order_gt0 -ltnNge ltnS leqn0. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)