<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def parseDTD(ExternalID, SystemID):
 """Load and parse an external subset. """

    ret = libxml2mod.xmlParseDTD(ExternalID, SystemID)
    if ret is None:raise parserError('xmlParseDTD() failed')
    return xmlDtd(_obj=ret)