code: @Override public void referenceSetUpdate(DoubleSolution solution) {
      if (refSet1Test(solution)) {
        for (DoubleSolution solutionInRefSet2 : referenceSet2) {
          double aux = SolutionUtils.distanceBetweenSolutionsInObjectiveSpace(solution, solutionInRefSet2);
          DoubleSolution auxSolution = solutionInRefSet2;
          if (aux < distanceToSolutionListAttribute.getAttribute(auxSolution)) {
            distanceToSolutionListAttribute.setAttribute(auxSolution, aux);
          }
        }
      } else {
        refSet2Test(solution);
      }
    }
docs: Update the reference set with a new solution
  @param solution