An constructor operation that builds a term from another (already defined) term. The constructed term has a root with constituent the identifier of the root of the original term. For instance an alien person from Mars (the PC way to say Martian) can be refined from person as follows: (define (person Martian) (planet Mars)). The root is Martian and has person as constituent. Refinement permits overriding, and in the example Martian overrides planet to be Mars--when formerly had been Earth. Thus, refinement does not imply subsumption. See § 2.2.2.