IndexedDocumentFactory is a factory of XML objects which
create indexed Element implementations to allow quicker lookup via name
of Element and Attributes though at the expense of more memory used
to create the name indexes.
UserDataDocumentFactory is a factory of XML objects which
support the adornment of a user data object on an Element or Attribute
instance such that the methods getData() and
setData() will get and set the values of a user data object.