Problem 1: F91 (100pts)

The f91 function is a recursive function, defined by the computer scientist John McCarthy as a test case for formal verification within computer science.

Write a recursive function f91 that takes a single argument n,

• returns n - 10 when n > 100
• returns f91(f91(n + 11)) when n ≤ 100

def f91(n):
    """Takes a number n and returns n - 10 when n > 100,
    returns f91(f91(n + 11)) when n ≤ 100.

    >>> f91(1)
    91
    >>> f91(2)
    91
    >>> f91(100)
    91
    """
    "*** YOUR CODE HERE ***"

Q: It seems that the argument of f91's recursive calls is not always simpler in terms of less-than relation of integers.

A: Oh, the meaning of "simpler" is not that simple ...