It must be emphasized, once again, that this selectionally introduced contextual feature cannot be arbitrary in a general convention regarding the forms of the grammar. Comparing these examples with their parasitic gap counterparts in (96) and (97), we see that this analysis of a formative as a pair of sets of features can be defined in such a way as to impose the levels of acceptability from fairly high (eg (99a)) to virtual gibberish (eg (98d)). For one thing, the systematic use of complex symbols raises serious doubts about the extended c-command discussed in connection with (34). Analogously, a subset of English sentences interesting on quite independent grounds suffices to account for the strong generative capacity of the theory. To characterize a linguistic level L, the descriptive power of the base component appears to correlate rather closely with the ultimate standard that determines the accuracy of any proposed grammar.
see also: WikiPedia -- Chomskybot