As we are considering each pair independently from the rest of the parse, this model does not correspond to a generative model of the kind standardly associated with PCFGs, but can be seen as a random field over the possible parses, with the features being the sequences and contexts (see (Abney, 1997)).
note that we were primarily interested in the clustering behavior, not the parsing behavior, and that the random field parameters have not been fit to any distribution over trees.