I'll describe some aspects of an ongoing project with Jeff Burdges, Steve Linton, Max Neunhoeffer and Richard Parker. We are using geometric ideas from small cancellation theory to develop a new class of practical algorithms for working with finite string-rewriting systems, working with a combinatorial generalisation of van Kampen diagrams. Amongst other applications, these can be used to solve the word problem in a wide variety of finitely-presented groups.