text: >>> Y = lambda f: lambda n: f(f,n)
  >>> fib = lambda f, n: None if n < 0 else (0 if n == 0 else (1 if n == 1 else f(f,n-1) + f(f,n-2))) #same as the first three implementations
  >>> [ Y(fib)(i) for i in range(-2, 10) ]
  [None, None, 0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
  