anchor.diff:  @response['Content-Length'] = body.length
   ```
   
  -The easiest way to write to the body is through the write method. `write()` will figure out the size of the string, calculate the Content-Length for us, and then set the header:
  +But the easiest way to write to the body is through the write method. `write()` will figure out the size of the string, calculate the Content-Length for us, and then set the header:
   
   ```ruby
   @response = Rack::Response.new
  
positive.code: 
  ## Rack::Response
  
  Rack::Response takes advantage of this pattern to provide response helpers. It has helpers for; setting status codes, setting headers, and writing to the body.
  
  <% note do %>
  
  The most useful powerful thing Rack::Response does is wrap the body in a proxy object, which means we can listen to it for writes. If we listen to the body for writes, we can update the response live -- very useful for streaming.
  
  <% end %>
  
  
negative.code: 
  # Responses
  
  The existence of Rack::Response is an enigma. After all, aren't our responses just an array? What good would a response object do when we have to return an array? The secret is that we only have to return something that _acts_ like an array. If we provide a `to_a` method we can return an object instead of an array.
  
  We can construct a valid response object like this:
  
  ```ruby
  class Response
  