receives events according to the DTD of the document.
The SAX parser may report these events in any order, regardless of the
order in which the notations and unparsed entities were declared; however,
all DTD events must be reported after the document handler's
startDocument event, and before the first startElement
event. It is up to the application to store the information for future
use (perhaps in a hash table or object tree). If the application encounters
attributes of type "NOTATION", "ENTITY", or "ENTITIES", it can use the
information that it obtained through this interface to find the entity
and/or notation that corresponds with the attribute value.
