The third feature of VW-CCG that we exploit is its ability to put grammar-specific restrictions on combinatory rules.
in part 2 of our construction (Section 3.1.3), we use rules whose secondary input categories contain a mix of variables and concrete categories, such as in Equation 16: Like the availability of composition rules of unbounded degree, the ability to use rule restrictions seems to be a very powerful feature, and one that perhaps most clearly sets VW-CCG apart from TAG.