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