define y<S..., T...> (S..., (S..., (S... -> T...) -> T...) -> T...):
  -> f; { f y } f call

define fac (Int32, (Int32 -> Int32) -> Int32):
  -> x, rec;
  if (x <= 1) { 1 } else { (x - 1) rec call * x }

define fib (Int32, (Int32 -> Int32) -> Int32):
  -> x, rec;
  if (x <= 2):
    1
  else:
    (x - 1) rec call -> a;
    (x - 2) rec call -> b;
    a + b

5  \fac y say  // 120
10 \fib y say  // 55