<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def copyDtd(self):
 """Do a copy of the dtd. """

        ret = libxml2mod.xmlCopyDtd(self._o)
        if ret is None:raise treeError('xmlCopyDtd() failed')
        __tmp = xmlDtd(_obj=ret)
        return __tmp