code: public static VectorAccumulator mkEuclideanNormAccumulator() {
          return new VectorAccumulator() {
              private BigDecimal result = BigDecimal.valueOf(0.0);
  
              @Override
              public void update(int i, double value) {
                  result = result.add(BigDecimal.valueOf(value * value));
              }
  
              @Override
              public double accumulate() {
                  double value = result.setScale(Vectors.ROUND_FACTOR, RoundingMode.CEILING).doubleValue();
                  result = BigDecimal.valueOf(0.0);
                  return Math.sqrt(value);
              }
          };
      }
docs: Makes an Euclidean norm accumulator that allows to use
  {@link org.la4j.Vector#fold(org.la4j.vector.functor.VectorAccumulator)} method for norm calculation.
  
  @return an Euclidean norm accumulator