We propose a very concise calculus for modelling object-oriented concepts such as objects, message sending, encapsulation and incremental modification. We show how to deal with recursion and provide some object-oriented examples. State and encapsulated inheritance are modelled by means of an incremental modification operator.
Mens, T., Mens, K., & Steyaert, P. (1995). OPUS: a Calculus for Modeling Object-Oriented Concepts. In Dilip Patel, Yuan Sun, Shushma Patel (ed.), Proceedings of Object-Oriented Information Systems (00IS 1994) (p. p. 152-165). Springer Verlag. https://doi.org/10.1007/978-1-4471-3016-1_12