def yes_no(msg):
    """Prompts the user for a yes/no reply.
       @param msg: Message to be displayed to the user.
       @return: Boolean True or False value based on user choice or None.
    """
    choice = input("\n%s (y/n): " % msg)
    if choice == "y":
        return True
    if choice == "n":
        return False
    return None