Nested Tree Recursion: fringe

Collect a list of the atoms on the fringe of a tree:

(define (fringe tree)
  (nreverse (fringeb tree '())))

(define (fringeb tree lst)
  (if (pair? tree)
      (fringeb (cdr tree)
               (fringeb (car tree) lst))
      (if (null? tree)
          lst
          (cons tree lst) ) ) )


> (fringe '(and (now)
                (for something
                     (completely different))))
(and now for something completely different)

Note that since fringeb uses cons to build its answer, the answer comes out backwards; therefore, fringe uses nreverse to reverse it.

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