anchor.diff:  
   -- The POST handler processes the form. If it is successful, it displays the
   -- parsed person. Otherwise, it displays the form again with error messages.
  -postPersonR :: Handler RepHtml
  +postPersonR :: Handler Html
   postPersonR = do
       ((result, widget), enctype) <- runFormPost personForm
       case result of
  
positive.code:                 <p>Invalid input, let's try again.
                  <form method=post action=@{PersonR} enctype=#{enctype}>
                      ^{widget}
                      <input type=submit>
              |]
  
  main :: IO ()
  
negative.code:     <*> aopt urlField "Website" Nothing
  
  -- The GET handler displays the form
  getHomeR :: Handler Html
  getHomeR = do
      -- Generate the form to be displayed
      (widget, enctype) <- generateFormPost personForm
  