Returns the url at the inputed index wihtin the stack.  If the index \
        is invalid, then a blank string is returned.
        
        :return     <str>

def urlAt(self, index):
        """
        Returns the url at the inputed index wihtin the stack.  If the index \
        is invalid, then a blank string is returned.
        
        :return     <str>
        """
        if 0 <= index and index < len(self._stack):
            return self._stack[index][0]
        return ''