If the position of the trace in (99c) were only relatively inaccessible to movement, the appearance of parasitic gaps in domains relatively inaccessible to ordinary extraction is to be regarded as nondistinctness in the sense of distinctive feature theory. Summarizing, then, we assume that the fundamental error of regarding functional notions as categorial cannot be arbitrary in a descriptive fact. Furthermore, relational information is not subject to the extended c-command discussed in connection with (34). For any transformation which is sufficiently diversified in application to be of any interest, this selectionally introduced contextual feature appears to correlate rather closely with a general convention regarding the forms of the grammar. Notice, incidentally, that the descriptive power of the base component is necessary to impose an interpretation on the requirement that branching is not tolerated within the dominance scope of a complex symbol.
see also: WikiPedia -- Chomskybot