<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _selector_as_string(self, selector):
 """ Returns a selector as a CSS string :param selector: A list of tinycss Tokens :type selector: list :returns: The CSS string for the selector :rtype: str """

        return ','.join(
            ''.join(token.as_css() for token in strip_whitespace(token_list))
            for token_list in split_on_comma(selector))