docs: def set(self, keys):
          """ Set new keys.
          Mind this will clear all attributes and keys before adding new keys
          
          >>> doc = Doc(docnum='1')
          >>> doc.terms = Text(multi=True, attrs={'tf': Numeric(default=1)}) 
          >>> doc.terms.add('copmputer', tf=12)
          >>> doc.terms.tf.values()
          [12]
          >>> doc.terms.set(['keyboard', 'mouse'])
          >>> list(doc.terms)
          ['keyboard', 'mouse']
          >>> doc.terms.tf.values()
          [1, 1]
          """
          # clear keys and atributes
          self._keys = OrderedDict()
          self.clear_attributes()
          _validate = self._ftype.validate
          for key in keys:
              self.add(_validate(key))