Powerdomains

If the meaning of a deterministic program may be considered to be a function from D to D, where D is some domain of "states", then it would seem that the meaning of a non-deterministic program is a function from D to 2D, or perhaps from 2D to 2D. To apply the methods of fixpoint semantics, then we should find some way to construe the power-set of a domain as itself a domain, with a suitable ordering