Technical report: Graph Neural Networks go Grammatical

This paper proposes a framework to formally link a fragment of an algebraic language to a Graph Neural Network (GNN). It relies on Context Free Grammars (CFG) to organise algebraic operations into generative rules that can be translated into a GNN layer model. Since the rules and variables of a CFG directly derived from a language contain redundancies, a grammar reduction scheme is presented making tractable the translation into a GNN layer. Applying this strategy, a grammar compliant with the third-order Weisfeiler-Lehman (3-WL) test is defined from MATLANG. From this 3-WL CFG, we derive a provably 3-WL GNN model called G$^2$N$^2$. Moreover, this grammatical approach allows us to provide algebraic formulas to count the cycles of length up to six and chordal cycles at the edge level, which enlightens the counting power of 3-WL. Several experiments illustrate that G$^2$N$^2$ efficiently outperforms other 3-WL GNNs on many downstream tasks.