anchor.diff:  <%= File.read("examples/include_manifestly_observable.rb") %>
   ```
   
  -=== All about observers
  +### All about observers
   
   *I'm still a little confused about `#on`. Is that just another way to add an observer?*
   
  
positive.code: 
  You can run the proof-of-concept benchmark for yourself! It's in the `scripts` directory.
  
  === Managing subscription lifetime
  
  *You know, it occurs to me that in the `Poem` example, it really doesn't make sense to have an `HtmlFormatter` plugged into a `Poem` forever. Is there a way to attach it before the call to `#recite`, and then detach it immediately after?
  
  
negative.code: 
  Nope! That's intentional. These are your classes, and I don't want to clutter up your API unnecessarily. `#on` and `#emit` are provided as conveniences for common actions. Anything else you need, you can get to via the `Emitter` returned from `#events`.
  
  ### Constraining event types
  
  *I see that un-handled events are just ignored. Doesn't that make it easy to miss events because of a typo in the name?*
  
  