Do U know Y this works?

# "Do U know Y this works?"

U = lambda f: f(f)

Y = lambda f: lambda n: f(Y(f))(n)

@U
def fact(g):
        return lambda n: (n < 2) and 1 or n * g(g)(n - 1)

print(fact(5))

@Y
def fib(g):
        return lambda n: (n >= 2) and g(n - 1) + g(n - 2) or n

print(fib(6))