  Tree-adjoining grammar (TAG) and combinatory categorial grammar (CCG) are two
well-established mildly context-sensitive grammar formalisms that are known to
have the same expressive power on strings (i.e., generate the same class of
string languages). It is demonstrated that their expressive power on trees also
essentially coincides. In fact, CCG without lexicon entries for the empty
string and only first-order rules of degree at most 2 are sufficient for its
full expressive power.
