code: def export(self):
          
          data = {}
          data["keys"] = dict( zip(self.keys(), range(len(self))) )
          # each attr
          for name in self._attrs.keys():
              data[name] = self.get_attribute(name).export()
          return data
docs: returns a dictionary pre-seriasation of the field
          
          :hide:
              >>> from pprint import pprint
          
          >>> from reliure.types import Text, Numeric
          >>> doc = Doc(docnum='1')
          >>> doc.terms = Text(multi=True, uniq=True, attrs={'tf': Numeric(default=1)}) 
          >>> doc.terms.add('chat')
          >>> doc.terms.add('rat', tf=5)
          >>> doc.terms.add('chien', tf=2)
          >>> pprint(doc.terms.export())
          {'keys': {'chat': 0, 'chien': 2, 'rat': 1}, 'tf': [1, 5, 2]}