public static Lst <T> cons <T>(T head, Lst <T> tail) =>
 tail.Insert(0, head);
 public static Lst <T> Cons <T>(this T head, Lst <T> tail) =>
 tail.Insert(0, head);
 public static Lst <A> Cons <A>(this A head, Lst <A> tail) =>
 tail.Insert(0, head);