def sum_digits(n):
    """Recursively calculate the sum of the digits of 'n'
    >>> sum_digits(7)
    7
    >>> sum_digits(30)
    3
    >>> sum_digits(228)
    12
    """
    """BEGIN PROBLEM 2.4"""
    return n % 10 + (0 if n < 10 else sum_digits(n // 10))
    """END PROBLEM 2.4"""