title: Angular 2 useExisting providers
body: <p>What are the usages for <code>useExisting</code> provider?</p>
  
  <p>Is it <code>useExistingOrThrowIfThereIsNone</code> or <code>useExistingOrCreateIfThereIsNone</code>? Can one of these behaviours be chosen on purpose somehow, depending on our needs? If one of them is not supported, can an unsupported one be emulated?</p>
  
  <p>The documentation is totally unclear on that and just gives an example that <code>useExisting</code> can reuse an instance from <code>useClass</code>.</p>