neumann-4.rkt

#lang racket

(require srfi/69)

(define neumann
  (let ((table (make-hash-table eqv?)))
    (hash-table-set! table 0 '())
    (lambda (n)
      (define (get key)
        (let ((val (hash-table-ref/default table key #f)))
          (if val
              key
              (get (- key 1)))))
      (define (put key)
        (if (= key n)
            (hash-table-ref table n)
            (let ((val (hash-table-ref table key)))
              (let ((key (+ key 1)))
                (hash-table-set! table key (cons val val))
                (put key)))))
      (put (get n)))))