Output event

An output event designates the output of a message. Normally there is a corresponding input event. The output event must come before the corresponding input event in time.

<message out area> ::= <message out symbol>

is attached to <instance axis symbol>

is attached to <message symbol>

<message out symbol> ::= <void symbol>

The <void symbol> is a geometric point without patial extension. The <message out symbol> is actually only a point which is on the instance axis. The end of the message symbol which has no arrow head is also on this point on the instance axis.