1. Visit the root

2. Visit all the nodes in the left subtree in preorder

3. Visit all the nodes in the right subtree in preorder

Desired behavior:

     (display-preorder bushy-tree)
     4   2   1   3   6   5   7

     (define display-preorder
       (lambda (tree)
         (cond ((empty-tree? tree))  ; do nothing
               (else (display (root tree))
                     (display "   ")
                     (display-preorder (left-subtree tree))
                     __?__ ))))