un-accum-and-reverse

;;https://stackoverflow.com/questions/62946821/
;;scheme-is-it-possible-to-convert-a-list-of-s-expressions-
;;into-a-list-of-atoms/62959036#62959036

;;by https://stackoverflow.com/users/5920214/tfb,
;;tweaked by https://stackoverflow.com/users/849891/will-ness,
;;removing the tailrec-accum-and-reverse
(define (tree->atoms tree)
  (define (atom? x) (not (pair? x)))
  (let tree->atoms-loop ([it tree]
                         [agenda '()])
    (cond [(null? it)
           ;; no more left
           (if (null? agenda)
               ;; no more agenda: we're done
               '()
               ;; more agenda, so carry on
               (tree->atoms-loop (first agenda)
                                 (rest agenda)))]
          [(atom? it)
           ;; we've found an atom which is not ()
           (if (null? agenda)
               ;; no more agenda: we're done
               (list it)
               ;; more agenda, so carry on
               (cons it (tree->atoms-loop (first agenda)
                                          (rest agenda))))]
          [else ; (not (atom? it))
           ;; cons: look at the car, and stuff the cdr onto the agenda
           (tree->atoms-loop (car it)
                             (cons (cdr it) agenda))])))