title: Convert list to group
body: <p>I have a question: i want to use a code like this:</p>
  
  <pre><code>var groups = shouldBeGroup ? items.GroupBy(p =&gt; p.Prop) : &lt;a group with all items&gt;
  foreach(var gr in groups)...
  </code></pre>
  
  <p>How to implement the second part? I would like a group with all items but GroupBy return many groups based on the specified property.</p>
  
  <p>Thanks.</p>