{str} containing messages to be
                logged.
              - C{system}: A C{str} which indicates the "system" which is
                generating this event.
              - C{isError}: A C{bool} indicating whether this event represents
                an error.
              - C{failure}: A L{failure.Failure} instance
              - C{why}: Used as header of the traceback in case of errors.
              - C{format}: A string format used in place of C{message} to
                customize the event.  The intent is for the observer to format
                a message by doing something like C{format % eventDict}.
        Nr