Pregroup grammars' generative capacity

There has been a movement within mathematical linguistics toward Lambek's pregroup grammars, which were mentioned in one or two earlier posts. The book Computational Algebraic Approaches to Natural Language (Casadio & Lambek eds., 2008) collects a number of papers on this subject, and this is recommended for those who wish to catch up on the trend. The book is also available as a free download directly from the publisher Polimetrica. Myself, I am somewhat ambivalent about the framework, but there does seem to be some confusion in the literature about its generative capacity.

The first paper in the mentioned volume is "Pregroup grammars and context-free grammars" by Buszkowski and Moroz. This paper relies on a result by Buszkowski (published in the Logical Aspects of Computational Linguistics proceedings in 2001) showing the weak equivalence between pregroup grammars and context-free grammars. Yet Greg Kobele and Marcus Kracht did a paper (unpublished) showing that pregroup grammars generate the recursively enumerable languages. I asked Greg about this, and he explained (as does his paper with Kracht) that key elements of the Buszkowski result are excluding the empty string from the context-free languages, and using only free pregroups. Kobele and Kracht showed, on the other hand, that by allowing all pregroups and also allowing the empty string, one achieves Turing-equivalence, generating all r.e. languages.

This is all esoteric stuff which is nevertheless important to have nailed down when one is working with a grammar formalism. Another issue with pregroup grammars is that they deny the existence of syntactic constituents in the normal sense. But that discussion has to wait for another post.