(define in-tree?
  (lambda (symbol tree-of-symbols)
    (if (not (pair? tree-of-symbols))
        (equal? tree-of-symbols symbol)
        (or (letrec ((branch-test
                      (lambda (branch)
                        (if (not (pair? branch))
                            (equal? symbol branch)
                            (if (pair? (car branch))
                                (branch-test (car branch))
                                (or (equal? (car branch) symbol)
                                    (branch-test (cdr branch))))))))
              (branch-test (car tree-of-symbols)))
            (in-tree? symbol (cdr tree-of-symbols))))))