Implementing angelic nondeterminism

We extend correctness and refinement reasoning methods in order to show how angelic nondeterminism can be systematically transformed into demonic nondeterminism or determinism. This kind of transformation is important because angelic nondeterminism assumes that an agent interested in establishing the postcondition will resolve the choices intelligently. When angelic nondeterminism is reduced into demonic nondeterminism or determinism, such intelligent choices are no longer necessary. Reducing angelic nondeterminism is generally not a refinement; however, when context is taken into consideration, it can be a correctness-preserving transformation.