title: Is there a Linq method to add a single item to an IEnumerable<T>?
body: <p>I am trying to do something like this:</p>
  <pre><code>image.Layers
  </code></pre>
  <p>which returns an <code>IEnumerable&lt;Layer&gt;</code> for all layers except the <code>Parent</code> layer, but in some cases, I just want to do:</p>
  <pre><code>image.Layers.With(image.ParentLayer);
  </code></pre>
  <p>because it's only used in a few places compared to the 100s of the usual usage which is satisfied by <code>image.Layers</code>. That's why I don't want to make another property that also returns the <code>Parent</code> layer.</p>