['vessel', 'open', 'fire', 'they', 'on']
["Synset('instrumentality.n.03')", 'establish-55.5', "Synset('recess.n.04')", '<PRP>', 'on']
However, an alien vessel opens fire on them just then.
However, an alien Synset('instrumentality.n.03') opens Synset('recess.n.04') on <PRP> just then.