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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 expire_at(self, key, expiry, **opts):
 """Set the explicit unix expiry time of a key."""

        key, store = self._expand_opts(key, opts)
        data = store.get(key)
        if data is not None:
            data = list(data)
            data[EXPIRY_INDEX] = expiry
            store[key] = tuple(data)
        else:
            raise KeyError(key)