code: def __create_preview_object_base(self, dct):
          
          if dct.get("_id"):
              del dct["_id"]
  
          preview_object_id = yield self.previews.insert(dct)
  
          raise Return(preview_object_id)
docs: The starting point for a preview of a future object.
          This is the object which will have future revisions iterated and applied to.
  
          :param dict dct: The starting object dictionary
          :return: The preview object id
          :rtype: str