returns the value of a given attribute for a given key
        
        >>> doc = Doc(docnum='1')
        >>> doc.terms = Text(multi=True, uniq=True, attrs={'tf': Numeric()}) 
        >>> doc.terms.add('chat', tf=55)
        >>> doc.terms.get_attr_value('chat', 'tf')
        55

def get_attr_value(self, key, attr):
        """ returns the value of a given attribute for a given key
        
        >>> doc = Doc(docnum='1')
        >>> doc.terms = Text(multi=True, uniq=True, attrs={'tf': Numeric()}) 
        >>> doc.terms.add('chat', tf=55)
        >>> doc.terms.get_attr_value('chat', 'tf')
        55
        """
        idx = self._keys[key]
        return self._attrs[attr][idx].get_value()