type option[T] = tag(none(), some(T));

type operator[T, U] = fn(&T) -> U;

fn option_map[T, U](&operator[T, U] f, &option[T] opt) -> option[U] {
  alt (opt) {
    case (some[T](x)) {
      ret some[U](f(x));
    }
    case (none[T]()) {
      ret none[U]();
    }
  }
}

fn id[T](&T x) -> T {
  ret x;
}

/* FIXME (issue #141):  See test/run-pass/constrained-type.rs.  Uncomment
 * the constraint once fixed. */
type rational = rec(int num, int den); // : _int.positive(*.den);

fn rational_leq(&rational x, &rational y) -> bool {
  // NB: Uses the fact that rationals have positive denominators WLOG.
  ret x.num * y.den <= y.num * x.den;
}