code: def indexOf(self, url):
          
          for i, (m_url, _) in enumerate(self._stack):
              if m_url == url:
                  return i
          return -1
docs: 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>