As a final aside, we note that the construction for extracting purely applicative derivations from the terms described by the RTG has interesting consequences for the weak generative capacity of PF-CCG.
it has the corollary that for any PF-CCG derivation δ over a string w, there is a permutation of w that can be accepted by a PF-CCG derivation that uses only application-that is, every string language L that can be generated by a PF-CCG grammar has a context-free sublanguage L such that all words in L are permutations of words in L .