code: def get_attr_value(self, key, attr):
          
          idx = self._keys[key]
          return self._attrs[attr][idx].get_value()
docs: 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