anchor.diff:    :markdown
       Angular reports an error if we neglect to list our custom pipe. 
       We didn't list the `DatePipe` in our previous example because all
  -    Angular built-in pipes are pre-registered by default. 
  +    Angular built-in pipes are pre-registered. 
       Custom pipes must be registered manually.
   :markdown
     If we are inclined to try this in a live-coding tool (such a [plunker](http://plnkr.co/)),
  
positive.code:   bootstrap(PowerBooster);
  
  :markdown
    Two things to note here:
    1. We use the pipe in the template expression exactly as we described in the pipe's comments. 
    We pass the value to transform from the left and give our pipe an exponent parameter of `10`.
    
  
negative.code:   
    Most pipes are stateless. The `DatePipe` in our first example is a stateless pipe. So is our custom `ExponentialStrengthPipe`.
    
    Stateful pipes are conceptually similar to classes in object-oriented programming. They can manage the data they transform. A pipe that creates an HTTP request, stores the response and displays the output, is a stateful pipe. 
    Pipes that retrieve or request data should be used cautiously, since working with network data tends to introduce error conditions that are better handled in JavaScript than in a template. 
    We can mitigate this risk by creating a custom pipe for a particular backend and bake-in the essential error-handling.
  
    ## The stateful `AsyncPipe`
    The Angular Async pipe is a remarkable example of a stateful pipe. 
  