Counting the leaves in a tree in Scheme (Lisp)

leaves_in_a_tree.scm

(define (count-leaves t)
  (cond ((null? t) 0)
        ((not (pair? t)) 1)
        (else (+ (count-leaves (car t)) 
        (count-leaves (cdr t))))))

; The general logic of this function is:
; If the tree is null, return 0
; elif: return 1 if at a leaf. We know
; That we're at a leaf if the position is not null and not a pair
; else: we are at a pair and must branch our function

; Testing it below:

1 ]=> (load "hierarchical_structures_1.scm")

;Loading "hierarchical_structures_1.scm"... done
;Value: count-leaves

1 ]=> (define tree-1 (cons (list 1 2) (list 3 4)))

;Value: tree-1

1 ]=> tree-1

;Value 11: ((1 2) 3 4)

1 ]=> (count-leaves tree-1)

;Value: 4

how to do a iterator version?