Rewriting modulo in diagrammatic algebras and application to categorification

We develop a rewriting theory suitable for diagrammatic algebras and lay down the foundations of a systematic study of their higher structures. In this paper, we focus on the question of finding bases. As an application, we give the first proof of a basis theorem for graded gl₂-foams, a certain diagrammatic algebra appearing in categorification and quantum topology. Our approach is algorithmic, combining linear rewriting, higher rewriting and rewriting modulo another set of rules -- for diagrammatic algebras, the modulo rules typically capture a categorical property, such as pivotality. In the process, we give novel approaches to the foundations of these theories, including to the notion of confluence. Other important tools include termination rules that depend on contexts, rewriting modulo invertible scalars, and a practical guide to classifying branchings modulo. This article is written to be accessible to experts on diagrammatic algebras with no prior knowledge on rewriting theory, and vice-versa.