This is an implementation of a low-level reactive engine (or toolkit),
which is mostly translated from SugarCubes (a Java toolkit).  The main
difference (aside from the implementation language) is that we support
preemption of actions (as in Berry's Communicating Reactive Processes
model).  This library is meant to be the target of higher-level reactive
models.

Information about SugarCubes can be found at

	http://www.inria.fr/meije/rc/SugarCubes/index.html