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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 _make_sid_selector(self, assets):
 """ Build an indexer mapping ``self.sids`` to ``assets``. Parameters assets : list[int] List of assets requested by a caller of ``load_raw_arrays``. Returns ------- index : np.array[int64] Index array containing the index in ``self.sids`` for each location in ``assets``. Entries in ``assets`` for which we don't have a sid will contain -1. It is caller's responsibility to handle these values correctly. """

        assets = np.array(assets)
        sid_selector = self.sids.searchsorted(assets)
        unknown = np.in1d(assets, self.sids, invert=True)
        sid_selector[unknown] = -1
        return sid_selector