parent_id could be zero

[Sy-Zhang]

I think the following line should be node.parent_id >= 0, since the parent_id is 0 for whose parent is the root node . Am I correct? https://github.com/timniven/cstlstm/blob/f9af6842f05432ba70062e4f4cbee14328d2acdf/cstlstm/tree_batch.py#L161