title: AQL return flat merged list
body: <p>I want to return a unique list of product options in AQL, I understand COLLECT can return unique list however, how do I now flatten it into an array.</p>
  
  <p>So I want to return the following filtered list: (lets call it list [A])</p>
  
  <pre><code>[
    [
      "Size"
    ],
    [
      "Size",
      "Color"
    ],
    [
      "value"
    ]
  ]
  </code></pre>
  
  <p>as: (lets call this list [B])</p>
  
  <pre><code>["Size","Color","Value"]
  </code></pre>
  
  <p>Query used to obtain list [A]</p>
  
  <pre><code>FOR product IN products
      COLLECT options = product.options[*].option
      FILTER LENGTH( options ) &gt; 0
  RETURN options
  </code></pre>
  
  <p>I tried FLATTEN, UNIQUE and no luck. Perhaps am not using functions accurately. My initial thought would be to repack items in list [A] to create list [B] something like push [A] into [B] if not in [B]</p>