['what', 'stop', 'he', 'EmptyParameter', 'EmptyParameter']
['what', 'stop-55.4-1-1', '<PRP>', 'EmptyParameter', 'EmptyParameter']
What stopped him was the thought of his sister, insisting on the fact that she was his sister as well as Trip.
what stopped <PRP> was the Synset('content.n.05') of <PRP> sister, insisting on the Synset('message.n.02') that <PRP> was <PRP> Synset('religious.n.01') as well as Synset('restraint.n.06').