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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def user_label(user: User) -> Optional[str]: """Given a user ORM FAB object, returns a label"""

    if user:
        if user.first_name and user.last_name:
            return user.first_name + ' ' + user.last_name
        else:
            return user.username

    return None