Infinite recursion detected... How do I prevent that?

I have a rule similar to this:

CREATE RULE rule_branch_delete AS
ON DELETE TO tree
DO DELETE
FROM tree
WHERE ancestor_id IS NOT NULL
AND OLD.child_id = ancestor_id;

The data is oraganized like this:

ancestor_id child_id
================================
NULL 1
1 2
1 3
1 4
1 5
1 6
1 7
1 8
2 4
2 5
2 6
3 7
3 8
etc.

This is an optimization table that relates all nodes in a branch to all
it's ancestor nodes; to it's parent, to it's grand parent to it's grand
grand parent, etc.

The tree is 3 to 5 levels deep. The intention is to delete all children
of a branch that share the same ancestor when an ancestor gets deleted.
This happens recursively, of course.

If I try a delete on the tree table I get "Infinite recursion detected
on rules on tree". I'm pretty sure it's not "infinite" in my case, how
can I make it delete the records regardless this "infinity"?

At the moment I use a trigger, but I would prefer a rule.

Alban.