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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def sortby(self, ntd):
 """Return function for sorting."""

        if 'reldepth' in self.grprobj.gosubdag.prt_attr['flds']:
            return [ntd.NS, -1*ntd.dcnt, ntd.reldepth]
        else:
            return [ntd.NS, -1*ntd.dcnt, ntd.depth]