Defines a DTD content model source. In other words, any object that
implements this interface is able to emit DTD content model "events"
to the registered DTD content model handler. These events could be
produced by parsing an XML document's internal or external subset,
could be generated from some other source, or could be created
programmatically. This interface does not say how the events
are created, only that the implementor is able to emit them.