code: public double add(int index, double delta) {
          if (isImmutable) 
              throw new UnsupportedOperationException(
                  "Cannot modify an immutable vector");
          int mapped = getIndex(index);
          double value = vector.getValue(mapped).doubleValue();
          vector.set(mapped, value + delta);
          return value + delta;
      }
docs: {@inheritDoc}