I am a dictionary holding only weakly on my values. Clients may expect to get a nil value for any object they request.

	adding
	add:

	accessing	top
	at:put: Set the value at key to be anObject. If key is not found, create a new entry for key and set is value to anObject. Answer anObject.

	adding	top
	add: Include newObject as one of the receiver's elements, but only if not already present. Answer newObject.