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]

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))