pick-toothless1.scm

(define (pick-toothless ls)
  (let rec ((ls ls)(acc '()))
    (if (or (null? ls)
            (null? (cdr ls)))
        (reverse acc)
        (let1 next (+ (car ls) 1)
          (if (= next (cadr ls))
              (rec (cdr ls) acc)
              (rec (cons next (cdr ls))
                   (cons next acc)))))))

(pick-toothless '(1 2 3 5 6 8 9))
;; (4 7)
(pick-toothless '(1 2 4 5 10))
;; (3 6 7 8 9)

pick-toothless2.scm

(use srfi-1)

(define (pick-toothless ls)
  (if (null? ls)
      '()
      (let ((min (car ls))(max (last ls)))
        (let1 pls (iota (+ (- max min) 1) min)
          (lset-difference = pls ls)))))

(pick-toothless '(1 2 3 5 6 8 9))
;; (4 7)
(pick-toothless '(1 2 4 5 10))
;; (3 6 7 8 9)

pick-toothless3.scm

(use gauche.collection)

(define (pick-toothless ls)
  (reverse!
   (fold2
    (^(e r i)(until(= i e)(push! r i)(inc! i))(values r(+ i 1)))
    '()
    (car ls)
    ls)))

(pick-toothless '(1 2 3 5 6 8 9))
;; (4 7)
(pick-toothless '(1 2 4 5 10))
;; (3 6 7 8 9)