code: public static VectorAccumulator mkManhattanNormAccumulator() {
          return new VectorAccumulator() {
              private double result = 0.0;
  
              @Override
              public void update(int i, double value) {
                  result += Math.abs(value);
              }
  
              @Override
              public double accumulate() {
                  double value = result;
                  result = 0.0;
                  return value;
              }
          };
      }
docs: Makes a Manhattan norm accumulator that allows to use
  {@link org.la4j.Vector#fold(org.la4j.vector.functor.VectorAccumulator)} method for norm calculation.
  
  @return a Manhattan norm accumulator