Problem space refinement
========================

A name for the problem space is usually sufficient for providing the
necessary context for problem solving in the subgoal.  When the problem space
is elaborated, it will often be with some abstract description of the
structure of its states and operators.  An explicit description of the
structure of the states or operators makes it possible to have general
techniques for manipulating these objects.  The best example of this arises
when a lookahead search is to be performed in the subgoal (see Section 
See Using lookahead search on page See Using lookahead search).