Convert the Selectors within this container to a string. This will just be a helper class for the subclasses that put their own name around the contents listed here.
<p>This implementation validates the container by calling verifySettings() and then validates each contained selector provided that the selector implements the validate interface.
Check to see whether any DataType we hold references to is included in the Stack (which holds all DataType instances that directly or indirectly reference this instance, including this instance itself).
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This selector just holds one other selector and forwards all
requests to it. It exists so that there is a single selector type that can exist outside of any targets, as an element of project. It overrides all of the reference stuff so that it works as expected. Note that this is the only selector you can reference.