Updater Objects
Subclass of Node.
This represents Node objects that update Updatable objects.
Updating can happen either by modifying the contents of an updatable
object, or by replacing it in the stream. An updater can have multiple
updatable objects associated with it.
u.getUpdatableCount() : int
Returns the number of Updatable nodes that this
is updating.
u.getUpdatables() : Iterator
Returns an iterator for the Updatable nodes that
this is updating.