Anonymous Y combination implementation for Python. Derived from https://rosettacode.org/wiki/Y_combinator#Python but more elegant

Y_combinator.py

Y = lambda f:(lambda y: f(lambda n: y(y)(n)))(lambda y: f(lambda n: y(y)(n)))
"""
Proof:
Y = lambda f:(lambda y: f(lambda n: y(y)(n)))
              (lambda y: f(lambda n: y(y)(n)))
Y(g) = (lambda y: g(lambda n: y(y)(n))
      (lambda y: g(lambda n: y(y)(n))
Y(g) = g(
    lambda n: (
        (lambda y: g(lambda n: y(y)(n)))
        (lambda y: g(lambda n: y(y)(n)))
        )(n))
    )
Y(g) = g(
    (lambda y: g(lambda n: y(y)(n)))
    (lambda y: g(lambda n: y(y)(n)))
    )
Y(g) = g(Y(g))
"""
# Factorial example
fac = lambda g:lambda i: 1 if i==1 else i*g(i-1)

# 6!
print(Y(fac)(6))