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Exercise solution:
Creation of association lists

; Return the association list formed by keys from key-list and values from value-list.
; Terminate when the shortest of the lists is exhausted.
(define (pair-up key-list value-list)
  (if (or (null? key-list) (null? value-list))
       (cons (car key-list) (car value-list))
       (pair-up (cdr key-list) (cdr value-list)))))