def _cook(self, object):
        """
        (internal) Backreference an object.

        Notes on this method for the hapless future maintainer: If I've already
        gone through the prepare/preserve cycle on the specified object (it is
        being referenced after the serializer is \"done with\" it, e.g. this
        reference is NOT circular), the copy-in-place of aList is relevant,
        since the list being modified is the actual, pre-existing jelly
        expression that was returned for that object. If not, it's technically
        superfluous, since the value in self.preserved didn't need to be set,
        but the invariant that self.preserved[id(object)] is a list is
        convenient because that means we don't have to test and create it or
        not create it here, creating fewer code-paths.  that's why
        self.preserved is always set to a list.

        Sorry that this code is so hard to follow, but Python objects are
        tricky to persist correctly. -glyph
        """
        aList = self.preserved[id(object)]
        newList = copy.copy(aList)
        # make a new reference ID
        refid = self._ref_id
        self._ref_id = self._ref_id + 1
        # replace the old list in-place, so that we don't have to track the
        # previous reference to it.
        aList[:] = [reference_atom, refid, newList]
        self.cooked[id(object)] = [dereference_atom, refid]
        return aList

        (internal) Backreference an object.

        Notes on this method for the hapless future maintainer: If I've already
        gone through the prepare/preserve cycle on the specified object (it is
        being referenced after the serializer is "done with" it, e.g. this
        reference is NOT circular), the copy-in-place of aList is relevant,
        since the list being modified is the actual, pre-existing jelly
        expression that was returned for that object. If not, it's technically
        superfluous, since the value in self.preserved didn't need to be set,
        but the invariant that self.preserved[id(object)] is a list is
        convenient because that means we don't have to test and create it or
        not create it here, creating fewer code-paths.  that's why
        self.preserved is always set to a list.

        Sorry that this code is so hard to follow, but Python objects are
        tricky to persist correctly. -glyph
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/spread/jelly.py