* This is similar to `treap_lookup', in that it returns the
* requested node if it already exists, or null otherwise, but it
* also records in P information to be used by `treap_insert' to
* This is similar to `treap_lookup', in that it returns the
* requested node if it already exists, or null otherwise, but it
* also records in P information to be used by `treap_insert' to