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- (IBAction)login:(id)sender {
    // This is an example of how to declare a block named loginHandler. It starts with a
    // return type, then the name of the block with a special syntax for naming. The next
    // set of parentheses hold the parameters and their types. Then assigning the block 
    // on the right side looks just like how we defined it in the video (carat with the
    // same parameters in parentheses).
    void (^loginHandler)(PFUser *user, NSError *error) = ^(PFUser *user, NSError *error) {
        if (error) {
            UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"Sorry!"
                                                                message:@"Error logging in."
                                                               delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil];
            [alertView show];
        }
        else {
            [self.navigationController popToRootViewControllerAnimated:YES];
        }
    };

    // Add your code below!
    [PFUser logInWithUsernameInBackground:@"test" password:@"test" block:loginHandler];

}