To describe systems composed of interacting parts, scientists and
engineers draw diagrams of networks: flow charts, Petri nets,
electrical circuit diagrams, signal-flow graphs, chemical reaction
networks, Feynman diagrams and the like. All these different diagrams
fit into a common framework: the mathematics of symmetric monoidal
categories. While originally the morphisms in such categories were
mainly used to describe processes, we can also use them to
describe open systems.