def decimalToRoman(index):
    """Converts an int to a roman numerical index"""
    assert isinstance(index, int) and index > 0
    roman = [(1000, 'M'), (900, 'CM'),
             (500, 'D'), (400, 'CD'),
             (100, 'C'), (90, 'XC'),
             (50, 'L'), (40, 'XL'),
             (10, 'X'), (9, 'IX'),
             (5, 'V'), (4, 'IV'),
             (1, 'I')]
    roman_num = ''
    while index:
        for value, num in roman:
            if index >= value:
                roman_num += num
                index -= value
                break
    return roman_num