(define scan-left
  (lambda (fn initial elements)
    (if (null? elements)
        initial
        (scan-left fn (fn initial elements) (cdr elements)))))
 
;;(scan-left (lambda (state current) (display current) (newline)) '() '(A B C))
 
(define delete-last-occurrence
  (lambda (elements token)
    ;; We're going to scan through the list, keeping a state consisting of
    ;;  - the cons cell *BEFORE* the (currently) last occurrence
    ;;  - the previous cons cell
    (define make-state
      (lambda (cell-b prev-cell)
        (cons cell-b prev-cell)))
    (define cell-before
      (lambda (state)
        (car state)))
    (define previous-cell
      (lambda (state)
        (cdr state)))
    ;; If the head of the current (sub)list is equal to the token,
    ;; then remember the previous cons cell as the cell before the
    ;; (currently) last occurrence.
    ;; Else just update the previous cell.
    (define consume-element
      (lambda (state current)
        (if (eqv? (car current) token)
            (make-state (previous-cell state) current)
            (make-state (cell-before state) current))))
    ;; Handle the easy cases separately.
    (cond ((null? elements) '())
          ((null? (cdr elements))
           (if (eqv? (car elements) token) '() elements))
          (else
           (let ((state
                  (scan-left consume-element 
                             (make-state '() elements)
                             (cdr elements))))
             (if (pair? (cell-before state))
                 ;; We found an occurrence, and the cell-before is the ... well 
                 ;; cell before that last occurrence. Hence we modify it such that
                 ;; its cdr no longer "points" to the cell whose car is the last
                 ;; occurrence but instead "skips" that element.
                 (let ((cell-to-modify (cell-before state)))
                   (set-cdr! cell-to-modify
                         (cdr (cdr (cell-before state))))
                   elements)
                 ;; No occurrence found. Either there is none, or only the first
                 ;; element is equal to the token.
                 (if (eqv? (car elements) token)
                     (cdr elements)
                     elements)))))))
 
 
(display (delete-last-occurrence '() 'A))(newline)
(display (delete-last-occurrence '(A) 'A))(newline)
(display (delete-last-occurrence '(A B) 'A))(newline)
(display (delete-last-occurrence '(A B A) 'A))(newline)
(display (delete-last-occurrence '(A B A B) 'A))(newline)
 
 
 

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