Suppose, for instance, that the theory of syntactic features developed earlier appears to correlate rather closely with the system of base rules exclusive of the lexicon. On the other hand, this analysis of a formative as a pair of sets of features does not readily tolerate irrelevant intervening contexts in selectional rules. To characterize a linguistic level L, the systematic use of complex symbols is unspecified with respect to the traditional practice of grammarians. Comparing these examples with their parasitic gap counterparts in (96) and (97), we see that this selectionally introduced contextual feature is, apparently, determined by the extended c-command discussed in connection with (34). It may be, then, that any associated supporting element is to be regarded as nondistinctness in the sense of distinctive feature theory.
see also: WikiPedia -- Chomskybot