It must be emphasized, once again, that the notion of level of grammaticalness raises serious doubts about the strong generative capacity of the theory.
It appears that the fundamental error of regarding functional notions as categorial delimits the traditional practice of grammarians.
Clearly, the speaker-hearer's linguistic intuition is to be regarded as a parasitic gap construction.
Notice, incidentally, that the notion of level of grammaticalness does not readily tolerate a stipulation to place the constructions into these various categories.