Terms with unbounded demonic and angelic nondeterminacy

We show how to introduce demonic and angelic nondeterminacy into the term language of each type in typical programming or specification language. For each type we introduce (binary infix) operators image and image on terms of the type, corresponding to demonic and angelic nondeterminacy, respectively. We generalise these operators to accommodate unbounded nondeterminacy. We axiomatise the operators and derive their important properties. We show that a suitable model for nondeterminacy is the free completely distributive complete lattice over a poset, and we use this to show that our axiomatisation is sound. In the process, we exhibit a strong relationship between nondeterminacy and free lattices that has not hitherto been evident.