anchor.diff:    
     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. An example would be a Pipe that creates an HTTP request, stores the response and displays the output. Pipes that actually retrieve or request data should be used cautiously, since working with network data tends to introduce error conditions which are better handled in JavaScript instead of template. This risk could be mitigated by creating custom pipe for a particular backend, which can bake in common error-handling strategies.
  +  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. 
  
positive.code:     `,
      pipes: [FetchJsonPipe]
    })
    class HeroesList implements AfterViewInit {
    }
  
    bootstrap(HeroesList);
  ```
  
negative.code:   bootstrap(PowerBooster);
  
  :markdown
    Two things to note:
    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`.
    
  