To provide a constituent structure for T(Z,K), the notion of level of grammaticalness cannot be arbitrary in a parasitic gap construction. For any transformation which is sufficiently diversified in application to be of any interest, a descriptively adequate grammar is not subject to the traditional practice of grammarians. Presumably, the speaker-hearer's linguistic intuition can be defined in such a way as to impose irrelevant intervening contexts in selectional rules. In the discussion of resumptive pronouns following (81), the systematic use of complex symbols does not readily tolerate the levels of acceptability from fairly high (eg (99a)) to virtual gibberish (eg (98d)). To characterize a linguistic level L, this selectionally introduced contextual feature appears to correlate rather closely with the requirement that branching is not tolerated within the dominance scope of a complex symbol.
see also: WikiPedia -- Chomskybot