proposition: Lemma except_path n h t : path (except n) h t -> path g h t.
proof: Proof. move/(pathP n) => H; apply/(pathP n) => i Hi; exact: (except_rel (H _ Hi)). Qed.
imports: mathcomp(all_ssreflect ssralg fingroup finalg perm zmodp) mathcomp(matrix) ssr_ext