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InsertableElementsInfo is a simple 'data packet' class that is used to
get information into and out of the validator APIs that allow you to ask
what kind of elements can be inserted into a particular place in an
element's content model.
Since a Document may contain at most one top-level Element child,
and at most one DocumentType declaraction, we need to subclass our
add-children methods to implement this constraint.
Indicates that one of the valid things after the insert point is
'end of content', which means that the element being inserted
after can legally be the last element.