parameters: (self, name, externalID, systemID)
docstring: called when a DOCTYPE declaration has been found, name is the
             DTD name and externalID, systemID are the DTD public and system
             identifier for that DTd if available
docstring_summary: called when a DOCTYPE declaration has been found, name is the
             DTD name and externalID, systemID are the DTD public and system
             identifier for that DTd if available
docstring_tokens: called when a DOCTYPE declaration has been found name is the DTD name and externalID systemID are the DTD public and system identifier for that DTd if available
function: def externalSubset(self, name, externalID, systemID):
          """called when a DOCTYPE declaration has been found, name is the
             DTD name and externalID, systemID are the DTD public and system
             identifier for that DTd if available"""
          pass
function_tokens: def externalSubset ( self , name , externalID , systemID ) : pass