code: public double foldNonZeroInColumn(int j, VectorAccumulator accumulator) {
          eachNonZeroInColumn(j, Vectors.asAccumulatorProcedure(accumulator));
          return accumulator.accumulate();
      }
docs: Folds non-zero elements of the specified column in this matrix with the given {@code accumulator}.
  
  @param j the column index.
  @param accumulator the {@link VectorAccumulator}.
  
  @return the accumulated value.