code: private static String getPrimaryKey(final String[] tuple, final int[] priKeyIndices,
        final char sep) {
      assert priKeyIndices.length < tuple.length;
      final StringBuilder sb = new StringBuilder();
      final int keys = priKeyIndices.length;
      for (int i = 0; i < keys; i++) {
        final int idx = priKeyIndices[i];
        sb.append(tuple[idx]);
        if ((i + 1) < keys) { sb.append(sep); }
      }
      return sb.toString();
    }
docs: also used by test