  Reactive systems are systems that maintain an ongoing interaction with their
environment, activated by receiving input events from the environment and
producing output events in response. Modern programming languages designed to
program such systems use a paradigm based on the notions of instants and
activations. We describe a library for Standard ML that provides basic
primitives for programming reactive systems. The library is a low-level system
upon which more sophisticated reactive behaviors can be built, which provides a
convenient framework for prototyping extensions to existing reactive languages.
