def strdigit_normalize(digit):
    """Normalizes input to format '0x'. Example: '9' -> '09'"""
    assert type(digit) is str, 'Invalid input. Must be a string.'
    s = int(digit)
    assert s >= 0, 'Invalid input. Must be string representing a positive number.'
    if s < 10:
      return '0' + str(s)
    return digit