title: Adding lists as elements to make a list of list
body: <p>For my assignment, I need to return a list of lists given a tree. <a href="https://i.stack.imgur.com/5wsNg.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/5wsNg.png" alt="enter image description here"></a>
  <a href="https://i.stack.imgur.com/9ZHkQ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/9ZHkQ.png" alt="enter image description here"></a></p>
  
  <p>For example, given this tree, we return the paths of the leaves, 0 being going to the left and 1 being going to the right. At the current moment, I believe my function makes sense; however, I have no idea how to make the lists into a list of lists. I believe an operation needs to be put where the question mark is; however I've looked online and I could not find anything. Is there an operation for this? Or do I have to create my own helper function? Thank you.</p>
  
  <pre><code>let rec dTree_path t = 
     match t with 
     | Leaf(x) -&gt; []
     | Node(x, lt, rt) -&gt; [ 0 :: (dTree_path lt) ] ?? [1 :: (dTree_path rt)];;
  </code></pre>
  
  <p>I </p>