Exercise 4.60: 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.