# [racket] I don't understand structural recursion.

Hello,
I should probably warn you that this is a homework question, but I have
tried to solve it, honestly.
"Construct a program that outputs (in some order) all structurally
distinct binary trees of n nodes. Two trees are considered structurally
distinct if they have different numbers of nodes or if their left or
right subtrees are structurally distinct."
Here's my attempt,
(define-struct tree (left right))
;; trees : Integral -> (ListOf Tree)
;; Return a list of all structurally distinct binary trees containing
;; N nodes.
(define (trees N)
(cond ((< N 1) empty)
((= N 1) (make-tree empty empty))
(else
(map (lambda (root)
(let ((left-tree (trees (- root 1)))
(right-tree (trees (- N 1 root))))
(make-tree left-tree right-tree)))
(build-list N add1)))))
I'm ashamed to point out that my function doesn't comply with my
contract. That's my problem: I end up inserting lists of trees instead
of structures into my tree's children. I can't see a away to avoid
making a list in the recursive calls, though.
I'm all tangled up with this. Can anyone help me out?
Thanks,
Damien.