Ensuring all nodes are transformed

[davetapley]

To simplify debugging and for future proofing it'd be useful for me to be able to have Transformer raise if any node isn't handled by the transform.

I presume that isn't supported right now?

[davetapley]

Never mind, this works:

class NodeTransformer(Transformer):
    def __default__(self, token, children, meta):
        raise NotImplementedError()