;; This procedure is equivalent to the two mutually recursive
;; procedures even-part and odd-part on page 184 from the book.
(define 2-way-split
  (lambda (which lst)
    (cond ((null? lst) '())
          ((equal? which 'odd)
           (cons (car lst) (2-way-split 'even (cdr lst))))
          ((equal? which 'even)
           (2-way-split 'odd (cdr lst)))
          (else (error "wrong argument in 2-way-split" which)))))

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;; We can write mutually recursive procedures to split a list into
;; three equal (well, as equal as possible) parts, as follows.

(define first-part
  (lambda (lst)
    (if (null? lst)
        '()
        (cons (car lst) (third-part (cdr lst))))))

(define third-part
  (lambda (lst)
    (if (null? lst)
        '()
        (second-part (cdr lst)))))

(define second-part
  (lambda (lst)
    (if (null? lst)
        '()
        (first-part (cdr lst)))))

;; Or equivalently as one procedure.
(define 3-way-split
  (lambda (which lst)
    (cond ((null? lst) '())
          ((equal? which 'first)
           (cons (car lst) (3-way-split 'third (cdr lst))))
          ((equal? which 'second)
           (3-way-split 'first (cdr lst)))
          ((equal? which 'third)
           (3-way-split 'second (cdr lst)))
          (else (error "wrong argument in 3-way-split" which)))))