docs: protected void checkMissing(Instances instances) throws Exception {
      for (int i = 0; i < instances.numInstances(); i++) {
        Instance ins = instances.instance(i);
        for (int j = 0; j < ins.numValues(); j++) {
          if (ins.index(j) != ins.classIndex())
            if (ins.isMissingSparse(j)) {
              throw new Exception("ERROR: KDTree can not deal with missing "
                  + "values. Please run ReplaceMissingValues filter "
                  + "on the dataset before passing it on to the KDTree.");
            }
        }
      }
    }