Returns the index of the inputed url for this stack.  If the url is \
        not found, then -1 is returned.
        
        :param      url | <str>
        
        :return     <int>

def indexOf(self, url):
        """
        Returns the index of the inputed url for this stack.  If the url is \
        not found, then -1 is returned.
        
        :param      url | <str>
        
        :return     <int>
        """
        for i, (m_url, _) in enumerate(self._stack):
            if m_url == url:
                return i
        return -1