title: Searching a tree using LINQ
body: <p>I have a tree created from this class.</p>
  
  <pre><code>class Node
  {
      public string Key { get; }
      public List&lt;Node&gt; Children { get; }
  }
  </code></pre>
  
  <p>I want to search in all children and all their children to get the ones matching a condition:</p>
  
  <pre><code>node.Key == SomeSpecialKey
  </code></pre>
  
  <p>How can I implement it?</p>