def system(tokens):
    """``system`` descriptor validation."""
    if len(tokens) > 2:
        return

    keyword = get_keyword(tokens[0])

    if keyword == 'extends':
        if len(tokens) == 2:
            second_keyword = get_keyword(tokens[1])
            if second_keyword:
                return (keyword, second_keyword, None)
    elif keyword == 'fixed':
        if len(tokens) == 1:
            return (None, 'fixed', 1)
        elif tokens[1].type == 'number' and tokens[1].is_integer:
            return (None, 'fixed', tokens[1].int_value)
    elif len(tokens) == 1 and keyword in (
            'cyclic', 'numeric', 'alphabetic', 'symbolic', 'additive'):
        return (None, keyword, None)
``system`` descriptor validation.
weasyprint/css/validation/descriptors.py