title: What is an efficient and elegant way to add a single element to an immutable set?
body: <p>I have an immutable set (cast as a <code>Set&lt;Integer&gt;</code>) that potentially contains many elements. I need a Collection that contains the elements from that set plus one additional element. I have kludgy code in place to copy the set, then append the element, but I'm looking for The Right Way that keeps things as efficient as possible.</p>
  
  <p>I have Guava available, though I do not require its use.</p>