A set of messages sent when an event is triggered. Holds only weakly to the receivers of those messages.

Instance variables:
receivers <WeakArray of: Object> weak receivers of messages.
messages <Array of: MessageSend> array of messages to be sent.

	enumerating
	do:

	initialize
	initialize:

	adding	top
	add: Add aMessageSend to the set of messages to be sent when the receiver is activated

	enumerating	top
	do: Evaluate aBlock with each of the receiver's elements as the argument.

	initialize	top
	initialize:

	instance creation	top
	new Answer a new instance of the receiver (which is a class) with no indexable variables. Fail if the class is indexable.
	new: Answer an instance of this class with the number of indexable variables specified by the argument, sizeRequested.