(define (list->pnp-tree los)
  (cond
    [(empty? los) empty]
    [(and (list? (third los))(list? (second los)))
     (make-pnp-tree (first los) (list->pnp-tree (second los)) (list->pnp-tree (third los)))]
    [(list? (second los)) (make-pnp-tree (first los) (list->pnp-tree (second los)) (third los))]
    [(list? (third los)) (make-pnp-tree (first los) (second los) (list->pnp-tree (third los)))]
    [else (make-pnp-tree (first los) (second los) (third los))]
    )
  )