def is_roman_numeral(s):
    """Return True if s is a roman numeral.
    
    s -- string
    """
    if not s:
        return False
    # there is a comma in the alphabet, because some categories are
    # combined, split by comma
    alphabet = dict.fromkeys("IVXDCM,")
    for i in s:
        if i not in alphabet:
            return False
    return True