code: public double foldColumn(int j, VectorAccumulator accumulator) {
          eachInColumn(j, Vectors.asAccumulatorProcedure(accumulator));
          return accumulator.accumulate();
      }
docs: Folds all elements of specified column in this matrix with given {@code accumulator}.
  
  @param j the column index
  @param accumulator the vector accumulator
  
  @return the accumulated value