def create_urgent_message(normal_message: str, invoking_user: str='') ->str:
    """consumes a string representing a message to be posted to a discord
channel and produces an urgent version by prepending a string
mentioning @everyone. optionally consumes a string representing the
name of the user that invoked this urgent message; if invoking_user is
set, the urgent message will blame that user.

    """
    if invoking_user:
        urgent_prefix = f'{invoking_user} asked me to tell @everyone that:'
    else:
        urgent_prefix = f'@everyone should know that'
    return '\n'.join([urgent_prefix, normal_message])
