TLZ

A Z schema can specify a functional system—one that produces an output in response to an input. For example, a simple text editor is a functional system; it can be specified by a Z schema that describes the effect of each keystroke on the screen. A reactive system interacts with its environment in a more complex fashion. Adding an interrupt key, which allows the user to stop a long operation before it completes, turns a simple text editor into a reactive system. Concurrent and distributed systems are usually best viewed as reactive systems. Z by itself is inadequate for specifying reactive systems.