A short proof that $O_2$ is an MCFL
This finding is of central importance for understanding the relation between extreme free word order and grammar classes in natural language syntax.
The paper tackles the problem of proving that O2 is a multiple context-free language by presenting a new proof that avoids complex geometric concepts, creating prospects for extending results to higher dimensions.
We present a new proof that $O_2$ is a multiple context-free language. It contrasts with a recent proof by Salvati (2015) in its avoidance of concepts that seem specific to two-dimensional geometry, such as the complex exponential function. Our simple proof creates realistic prospects of widening the results to higher dimensions. This finding is of central importance to the relation between extreme free word order and classes of grammars used to describe the syntax of natural language.