More complete occurrence management.

[gallandarakhneorg]

Several modifiers or API tools may be included in order to control the event handling.
For example:

	val uniq = new AtomicBoolean(false)
	on Ready {
		if (!this.uniq.getAndSet(true)) {
			// Do Something
		}
	}

This code enables the agent to react to Ready only for the first first time it is received.

Several ways to handle events may be:

• react ot the first occurrence only
• several on handlers are exclusive independently of their guards
• following a complex interaction protocol