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=
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''.