This tests whether name is a legitimate tag to use as a
direct sub-element of the element tag according to the
definition of the element element in the specified DTD. This
is a generic function that dispatches on the element type, so that
different version take effect for XMLSequenceContent,
XMLOrContent, XMLElementContent.
Since parseDTD no longer works, this function isn't
expected to work either and will be removed in a future version of
XML.