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]}

def export(self):
        """ 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]}
        """
        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