The study of abstract computing processes by means of various formal systems and calculuses. An early influential calculus was the calculus of communicating systems (CCS) of R. Milner. This has given rise to many adaptations and new approaches to a theory of processes. For example, it led to the systematic development of process algebra to which many process calculuses may be said to belong, and it inspired an influential reformulation of the parallel language CSP as a process calculus by C. A. R. Hoare.