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