Problem 1: Number of Six (100pts)

Write a recursive function number_of_six that takes a positive integer x and returns the number of times the digit 6 appears in x.

Use recursion - the tests will fail if you use any assignment statements.

def number_of_six(n):
    """Return the number of 6 in each digit of a positive integer n.

    >>> number_of_six(666)
    3
    >>> number_of_six(123456)
    1
    >>> from construct_check import check
    >>> # ban all assignment statements
    >>> check(HW_SOURCE_FILE, 'number_of_six',
    ...       ['Assign', 'AugAssign'])
    True
    """
    "*** YOUR CODE HERE ***"