The recursions we have seen so far have involved linear structures, that is, lists in which only the top level was involved. A second class of recursive programs operates on both halves of a pair; in general, such functions call themselves twice.

; Copy a tree structure
(define (copy-tree tree)
  (if (pair? tree)
      (cons (copy-tree (car tree))
            (copy-tree (cdr tree)))
      tree))

; Test equality of structure
(define (equal? x y)
  (if (pair? x)
      (and (pair? y)
           (equal? (car x) (car y))
           (equal? (cdr x) (cdr y)))
      (eqv? x y)) )

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