Problem 60
By giving the query (lives-near ?person (Hacker Alyssa P)) Alyssa P. Hacker is able to find people who live near her, with whom she can ride to work. On the other hand, when she tries to find all pairs of people who live near each other by querying (lives-near ?person-1 ?person-2) she notices that each pair of people who live near each other is listed twice; for example, (lives-near (Hacker Alyssa P) (Fect Cy D)) (lives-near (Fect Cy D) (Hacker Alyssa P)) Why does this happen? Is there a way to find a list of people who live near each other, in which each pair appears only once? Explain.
Problem 77
In section \(4.4 .3\) we saw that not and lisp-value can cause the query language to give "wrong" answers if these filtering operations are applied to frames in which variables are unbound. Devise a way to fix this shortcoming. One idea is to perform the filtering in a "delayed" manner by appending to the frame a "promise" to filter that is fulfilled only when enough variables have been bound to make the operation possible. We could wait to perform filtering until all other operations have been performed. However, for efficiency's sake, we would like to perform filtering as soon as possible so as to cut down on the number of intermediate frames generated.
