title: How to concatenate two IEnumerable<T> into a new IEnumerable<T>?
body: <p>I have two instances of <code>IEnumerable&lt;T&gt;</code> (with the same <code>T</code>). I want a new instance of <code>IEnumerable&lt;T&gt;</code> which is the concatenation of both.</p>
  <p>Is there a built-in method in .NET to do that or do I have to write it myself?</p>