title: How to write publisher that conditionally publish?
body: <p>Lets say I have a publisher like this:</p>
  
  <pre><code>NotificationCenter.default.publisher(for: NSNotification.Name.NSManagedObjectContextObjectsDidChange)
    .map { notification in /.. do something ../}
  </code></pre>
  
  <p>how do I make it so that in my block, I stop the publisher from emitting? </p>
  
  <p>I could make it a trymap and then throw an error. But it's so weird to have to throw just to indicate a lack of results (in this case, let's say a non relevant NSManagedObject change)</p>