code: long getValueAtPosition(final int pos) {
      if (itemsSeen_ == 0) {
        throw new SketchesArgumentException("Requested element from empty reservoir.");
      } else if ((pos < 0) || (pos >= getNumSamples())) {
        throw new SketchesArgumentException("Requested position must be between 0 and "
            + (getNumSamples() - 1) + ", inclusive. Received: " + pos);
      }
  
      return data_[pos];
    }
docs: Useful during union operations to avoid copying the items array around if only updating a few
  points.
  
  @param pos The position from which to retrieve the element
  @return The value in the reservoir at position <tt>pos</tt>