Higher dimensional multigraphs

We introduce the notion of higher dimensional multigraph. This notion extends that of multigraph, which underlies multicategories and is essentially equivalent to the notion of context-free grammar. We develop the definition and explain how it gives a semantically coherent category theoretic approach to the notion of higher order context-free grammar. It also gives a conceptual framework in which one can study rewrites, and rewrites of rewrites, etcetera, for proofs of sequent calculus. The definition involves a subtle interaction between geometry and linearly defined syntax; we explore the latter here, outlining the geometric intuition.