title: merge 2 collections, prefer items of the second collection
body: <p>I have 2 collections of objects. the objects have an object-type 'key' property (I use object.ReferenceEquals to test the key). how do I merge collection A and B to produce a distinct collection C that has all the items of A and B but not those of A that have a matching key with an item in B?</p>
  
  <p><strong>EDIT:</strong> mind you, the collections might also be empty.</p>