Before PopBeast can use the above information, it has to replace D and S below
[ square(D,S),door(D) ],with a specific square and door. It does this by matching against the proposition
square(door(2), [8,4])in its world model, giving S=
[8,4] and D=door(2). This
process is called finding the referent of D and S. As I hinted in
my discussion of ``the'', there are better ways to do this: PopBeast's
method is a very poor approximation to what any decent
language-understanding system should do, and to what we do. The problem
of finding referents also comes up in interpreting pronouns, and was one
of the objectives behind Shrdlu - consider a sentence like ``Pick up
the red block and put it on the large green box''.