sicp-ex-4.60

  
  
 because all the answers satisfy the rule. 
 we can sort the person in alphabetic order, then get only one pair. 
 (define (person->string person) 
   (if (null? person) 
       "" 
       (string-append (symbol->string (car person)) (person->string (cdr person))))) 
 (define (person>? p1 p2) 
   (string>? (person->sring p1) (person->string p2))) 
  
 (assert! (rule (asy-lives-near ?person1 ?person2) 
                (and (address ?person1 (?town . ?rest-1)) 
                        (address ?person2 (?town . ?rest-2)) 
                        (lisp-value person>? ?person1 ?person2))))