4.60 Exercise 4.60

Consider the rule lives-near, given as an example earlier:

(rule (lives-near ?person-1 ?person-2)

(and (address ?person-1 (?town . ?rest-1))

(address ?person-2 (?town . ?rest-2))

(not (same ?person-1 ?person-2))))

Alyssa P. Hacker has noticed that this will return pairings twice if both persons are parametized at once. This is a fairly straightforward consequence – if Alyssa and Cy live near each other, then by the rule body above, (lives-near (Hacker Alyssa P) (Fect Cy D)) and (lives-near (Fect Cy D) (Hacker Alyssa P)) are obviously both true. What this means is that lives-near is a commutative rule.

The rule needs a new condition strictly for removing duplicate pairs. One potential choice is a lexicographical (that is, alphabetical) comparison of the names – you could pass a function computing whether a name is alphabetically precedent to another to lisp-value and discard duplicate entries that way. It also isn’t unreasonable to have all the results be presented with names in alphabetical order.