(define (left-child tree)
  (cadr tree))
(define (left-child-name tree)
  (car (left-child tree)))
(define (right-child tree)
  (caddr tree))
(define (right-child-name tree)
  (car (right-child tree)))
(define (cur-name tree)
  (car tree))

(define (to-string phon-tree node-nbr child next-nbr)
  (let ((this-name (string-append "node"
                                  (number->string node-nbr)))
        (child-name (string-append "node"
                                   (number->string next-nbr))))
    (string-append child-name
                   "[label=\""
                   (cur-name child)
                   "\"];"
                   this-name
                   "--"
                   child-name
                   ";\n"
                   (to-dot-main child next-nbr))))

(define (to-dot-main phon-tree node-nbr)
  (let* ((this-name (string-append (number->string node-nbr)
                                  "node")))
    (if (null? (left-child phon-tree))
        ""
        (let ((x (* 2 (+ 1 node-nbr)))
              (y (* 2 (+ 2 node-nbr))))
          (string-append (to-string phon-tree node-nbr
                                    (left-child phon-tree)
                                    x)
                         (to-string phon-tree node-nbr
                                    (right-child phon-tree)
                                    y))))))

(define (to-dot phon-tree)
  (if (null? phon-tree)
      "\n"
      (string-append
       "graph{\n"
       "node[shape=plaintext];\n"
       "graph[bgcolor=\"transparent\"];\n"
       "node1[label=\""
       (cur-name phon-tree)
       "\"];\n"

       (to-dot-main phon-tree 1)
       "}")))

(define vowel-tree
  '("[vowel]"
    ("[back]"
     ("[tense]" () ())
     ("{tense}" () ()))
    ("{back}"
     ("[rnd]"
      ("[tense]" () ())
      ("{tense}"
       ("[high]" ())
       ("{high}" ())))
     ("{rnd}"
      ("[tense]"
       ("[high]" () ())
       ("{high}" () ()))
      ("{tense}"
       ("[low]" () ())
       ("{low}" () ()))))))
(define vowel2-tree
  '("[vowel]"
    ("[back]"
     ("[tense]"
      ("[high]" () ())
      ("{high}" () ()))
     ("{tense}"
      ("[high]" () ())
      ("{high}" () ())))
    ("{back}"
     ("[tense]"
      ("[high]" () ())
      ("{high}" () ()))
     ("{tense}"
      ("[high]" () ())
      ("{high}" () ())))))

(define cons-tree
  '("[cons]"
    ("[son]"
     ("[dor]"
      ("[high]" () ())
      ("{high}" () ()))
     ("{dor}"
      ("[cor]" () ())
      ("{cor}" () ())))
    ("{son}"
     ("[dor]"
      ("[voice]"
       ("[high]" () ())
       ("{high}" () ()))
      ("{voice}"
       ("[high]" () ())
       ("{high}" () ())))
     ("{dor}"
      ("[voice]"
       ("[cor]" () ())
       ("{cor}" () ()))
      ("{voice}"
       ("[cor]" () ())
       ("{cor}" () ()))))))

(display (to-dot cons-tree)) (newline)