anchor.diff:  Before redirecting to the sign up page, `ProviderSignInController` collects some information about the authentication attempt. 
   This information can be used to prepopulate the sign up form and then, after successful sign up, to establish a connection between the new account and the provider account.
   
  -To prepopulate the sign up form, you can fetch the user profile data from a connection retrieved from `ProviderSignInUtils.getConnection()`.
  +To prepopulate the sign up form, you can fetch the user profile data from a connection retrieved from `ProviderSignInUtils#getConnection()`.
   For example, consider this Spring MVC controller method that setups up the sign up form with a `SignupForm` to bind to the sign up form:
   
   ```java
  +@Autowired
  +ProviderSignInUtils providerSignInUtils;
  +
   @RequestMapping(value="/signup", method=RequestMethod.GET)
   public SignupForm signupForm(WebRequest request) {
  -    Connection<?> connection = ProviderSignInUtils.getConnection(request);
  +    Connection<?> connection = providerSignInUtils.getConnection(request);
       if (connection != null) {
           return SignupForm.fromProviderUser(connection.fetchUserProfile());
       } else {
  
positive.code: Twitter, for example, does not provide a user's email address, so the `getEmail()` method will always return null after a sign in attempt with Twitter.
  
  After the user has successfully signed up in your application a connection can be created between the new local user account and their provider account. 
  To complete the connection call `ProviderSignInUtils.handlePostSignUp()`. 
  For example, the following method handles the sign up form submission, creates an account and then calls `ProviderSignInUtils.handlePostSignUp()` to complete the connection:
  
  ```java
  @RequestMapping(value="/signup", method=RequestMethod.POST)
  
negative.code:     Account account = createAccount(form, formBinding);
      if (account != null) {
          SignInUtils.signin(account.getUsername());
          ProviderSignInUtils.doPostSignUp(account.getUsername(), request);
          return "redirect:/";
      }
      return null;
  