; Various versions of in-tree that I did not like.

(define in-tree-1?
  (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-1? symbol (cdr tree-of-symbols))))))

; Test with (in-tree-1? 'a (cons (cons (cons 'b null) 'a) null))

(define in-tree-2?
  (lambda (symbol tree-of-symbols)
    (if (not (pair? tree-of-symbols))
        (eq? symbol tree-of-symbols)
        (let ((left (equal? symbol (car tree-of-symbols)))
              (right (in-tree-2? symbol (cdr tree-of-symbols))))
          (or left right)))))

; Test with (in-tree-2? 'a (cons (cons 'a 'b) null))

(define in-tree-3?
  (lambda (symbol tree-of-symbols)
    (if (not (pair? tree-of-symbols))
        (eq? symbol tree-of-symbols)
        (let ((left (in-tree-3? symbol (car tree-of-symbols)))
              (right (in-tree-3? symbol (cdr tree-of-symbols))))
          (or left right)))))

; Inefficient to look at both subtrees

(define in-tree-4?
  (lambda (symbol tree-of-symbols)
    (if (not (pair? tree-of-symbols))
        (eq? symbol tree-of-symbols)
        (or (in-tree-4? symbol (car tree-of-symbols))
            (in-tree-4? symbol (cdr tree-of-symbols))))))