In <cite>(Johnson, 2002 )</cite> <cite>the author</cite> notes that the biggest weakness of <cite>the algorithm</cite> seems to be that it fails to robustly distinguish co-indexed and free empty nodes and it is lexicalization that may be needed to solve this problem. Moreover, <cite>the author</cite> suggests that <cite>the algorithm</cite> may suffer from overlearning, and using more abstract "skeletal" patterns may be helpful to avoid this.