Replace custom adjoin with cons

Duplicates in the seen list don't matter

3_16.rkt

@@ -15,22 +15,16 @@
          1)))
 
 (define (count-unique-pairs l)
-  (define (adjoin p pairs)
-    (cond ((null? pairs) (list p))
-          ((not (pair? p)) pairs)
-          ((eq? p (car pairs)) pairs)
-          (else (cons (car pairs)
-                      (adjoin p (cdr pairs))))))
   (trace-define (count l seen-pairs)
     (cond ((not (pair? l)) 0)
           ((memq l seen-pairs) 0)
           (else
            (+ (count (car l)
-                     (adjoin l seen-pairs))
+                     (cons l seen-pairs))
               (count (cdr l)
-                     (adjoin l
-                             (adjoin (car l)
-                                     seen-pairs)))
+                     (cons l
+                           (cons (car l)
+                                 seen-pairs)))
               1))))
   (count l '()))