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