(define (build-trees trees lefts rights)
  (rbtree-fold 
    (lambda (l tree-l)
      (rbtree-fold (lambda (r tree-r)
		     (tree-put n-lt n-gt 
			       (par r l) 
			       (tree-put n-lt n-gt (seq r l) tree-r)))
		   tree-l
		   rights))
    trees
    lefts))