Changed order of argout to [dump,model].
Noted that this is different than (Goldberg and Nivre, 2013) who allow
multiple root-children.
Goldberg and Nivre 2013 version of archybrid; allows multiple root children.
Noted that we modified valid_moves to output a single root-child.
Goldberg and Nivre 2013 version of arceager; allows multiple root children.