How to use the cssselect.parser.parse_series function in cssselect

def series(css):
            selector, = parse(':nth-child(%s)' % css)
            args = selector.parsed_tree.arguments
            try:
                return parse_series(args)
            except ValueError:
                return None

def series(css):
            selector, = parse(':nth-child(%s)' % css)
            args = selector.parsed_tree.arguments
            try:
                return parse_series(args)
            except ValueError:
                return None

def xpath_nth_child_function(self, xpath, function, last=False,
                                 add_name_test=True):
        try:
            a, b = parse_series(function.arguments)
        except ValueError:
            raise ExpressionError("Invalid series: '%r'" % function.arguments)
        if add_name_test:
            xpath.add_name_test()
        xpath.add_star_prefix()
        if a == 0:
            if last:
                b = 'last() - %s' % b
            return xpath.add_condition('position() = %s' % b)
        if last:
            # FIXME: I'm not sure if this is right
            a = -a
            b = -b
        if b > 0:
            b_neg = str(-b)
        else:

def xpath_nth_child_function(self, xpath, function, last=False,
                                 add_name_test=True):
        try:
            a, b = parse_series(function.arguments)
        except ValueError:
            raise ExpressionError("Invalid series: '%r'" % function.arguments)
        if add_name_test:
            xpath.add_name_test()
        xpath.add_star_prefix()
        if a == 0:
            if last:
                b = 'last() - %s' % b
            return xpath.add_condition('position() = %s' % b)
        if last:
            # FIXME: I'm not sure if this is right
            a = -a
            b = -b
        if b > 0:
            b_neg = str(-b)
        else:

def xpath_nth_child_function(self, xpath, function, last=False,
                                 add_name_test=True):
        try:
            a, b = parse_series(function.arguments)
        except ValueError:
            raise ExpressionError("Invalid series: '%r'" % function.arguments)

        # From https://www.w3.org/TR/css3-selectors/#structural-pseudos:
        #
        # :nth-child(an+b)
        #       an+b-1 siblings before
        #
        # :nth-last-child(an+b)
        #       an+b-1 siblings after
        #
        # :nth-of-type(an+b)
        #       an+b-1 siblings with the same expanded element name before
        #
        # :nth-last-of-type(an+b)
        #       an+b-1 siblings with the same expanded element name after