A geometric hierarchy beyond context-free languages

This paper defines a geometric hierarchy of language classes the first member of which is context-free languages. This hierarchy generalizes the difference between context-free languages and the class of languages generated by four weakly equivalent grammar formalisms that are of interest to computational linguists. A grammatical characterization of the hierarchy is given using a variant of control grammars. Each member of the progression is shown to share many of the attractive features of context-free grammars, in particular, we show that each member is an abstract family of languages. We give a progression of automata and show that it corresponds exactly to the language hierarchy defined with control grammars.