'Compute a two-row array of indices:
- the first row contains indices for the primary table,
- the second row contains the matching rows in the extra table or -1'

def _augment_indices(self, var_data, extra_map, as_string):

    data = self.data
    extra_map = dict(extra_map)
    if (np.nan in extra_map):
        del extra_map[np.nan]
    keys = (extra_map.get(val, (-1)) for val in self._values(data, var_data, as_string))
    return np.vstack((np.arange(len(data), dtype=np.int64), np.fromiter(keys, dtype=np.int64, count=len(data))))
