*
   * @since 20.0
   */
  public static final class LinearTransformationBuilder {

    private final double x1;
    private final double y1;

    private LinearTransformationBuilder(double x1, double y1) {
      this.x1 = x1;
      this.y1 = y1;
    }

    /**
     * Finish building an instance which also maps {@code x = x2} to {@code y = y2}. These values

        Map<String, Object> responseDataMap = new HashMap<>();
        responseDataMap.put("y.overwrite", "Y1");

        indexUpdateCallback.mergeResponseData(dataMap, responseDataMap);

        assertFalse(dataMap.containsKey("y.overwrite"));
        assertEquals(1, dataMap.size());
        assertEquals("Y1", dataMap.get("y"));
    }