code: public double add(int[] indices, double val) {
          checkIndices(indices);
          int c = getConfigIdx(indices);
          return values.set(c, s.plus(values.get(c), val));
      }
docs: Adds the value to the entry corresponding to the given indices.
  @param indices The indices of the multi-dimensional array.
  @param val The value to add.
  @return The previous value.