accessing
newestOf: userName 

	| user |
	user := (TwitterUser storedInstances select: [ :each | each userName = userName ]) first.
	self messages ifNil: [ ^ nil ].
	self messages ifEmpty: [ ^ nil ].
	user messages: self.
	^ ((user tweets)
		select: [ :tweet | tweet created = ((self collect: [ :each | each created ]) asSortedCollection last)]) first.