code: public double[] foldNonZeroInColumns(VectorAccumulator accumulator) {
          double[] result = new double[columns];
  
          for (int j = 0; j < columns; j++) {
              result[j] = foldNonZeroInColumn(j, accumulator);
          }
  
          return result;
      }
docs: Folds non-zero elements (in a column-by-column manner) of this matrix with given {@code accumulator}.
  
  @param accumulator the matrix accumulator
  
  @return the accumulated vector