title: Groovy 2d array
body: <p>I want to have a collection like this</p>
  
  <pre><code>[item1: [123, 123, 2321], item2: [1231,1222,1313]]
  </code></pre>
  
  <p>I tried using a map like this <code>[  : [] ]</code> but this is not allowed. How can I achieve the following structure. I want to add element using a string as a key: </p>
  
  <pre><code>content['item1'] &lt;&lt; 123
  </code></pre>