title: Combine two lists into one list Racket
body: <p>I have a question:</p>
  
  <p>I have two list of numbers for example (list1 3 6 7) and (list2 1 6 4 7). Now I have to combine both into (list3 1 3 4). So 6 and 7 are both in list1 and list2. List3 contains all numbers which occur only once. I hope u get what i mean if not just ask me :s!
  Here my start:</p>
  
  <pre><code>(define (diff list1 list2)
    (cond
      [(empty? list1) list2] ;; If list1 was empty return directly list2
      [(empty? list2) list1] ;; If list2 was empty return directly list1
      [else
        (???
  </code></pre>
  
  <p>I know that I have to compare first list1 with every number in list2 and then recursiv again. But how do I programm it?</p>