However, to ensure high quality of learned phrases, strict syntactic constraints were enforced at both the phrasal and sentential level.
they only considered event expressions and purpose phrases as verb phrases in two types (Figure 2(a)), the first phrasal type is a verb followed by the head noun of its direct object and the second phrasal type is a verb with an attached prepositional phrase, while reasonably both event expressions and purpose phrases can exist in many other syntactic structures.