If the position of the trace in (99c) were only relatively inaccessible to movement, this selectionally introduced contextual feature is rather different from the ultimate standard that determines the accuracy of any proposed grammar. Furthermore, any associated supporting element appears to correlate rather closely with the extended c-command discussed in connection with (34). Of course, a descriptively adequate grammar is, apparently, determined by the levels of acceptability from fairly high (eg (99a)) to virtual gibberish (eg (98d)). Nevertheless, the natural general principle that will subsume this case does not affect the structure of nondistinctness in the sense of distinctive feature theory. So far, the speaker-hearer's linguistic intuition can be defined in such a way as to impose irrelevant intervening contexts in selectional rules.
see also: WikiPedia -- Chomskybot