code: public void update(final ReservoirLongsSketch sketchIn) {
      if (sketchIn == null) {
        return;
      }
  
      final ReservoirLongsSketch rls =
          (sketchIn.getK() <= maxK_ ? sketchIn : sketchIn.downsampledCopy(maxK_));
  
      // can modify the sketch if we downsampled, otherwise may need to copy it
      final boolean isModifiable = (sketchIn != rls);
      if (gadget_ == null) {
        createNewGadget(rls, isModifiable);
      } else {
        twoWayMergeInternal(rls, isModifiable);
      }
    }
docs: Union the given sketch.
  <p>
  This method can be repeatedly called. If the given sketch is null it is interpreted as an empty
  sketch.
  </p>
  
  @param sketchIn The incoming sketch.