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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 _get_taxids(self, taxids=None):
 """Return user-specified taxids or taxids in self.taxid2asscs"""

        taxid_keys = set(self.taxid2asscs.keys())
        return taxid_keys if taxids is None else set(taxids).intersection(taxid_keys)