code: public void update(final Memory mem, final ArrayOfItemsSerDe<T> serDe) {
      if (mem != null) {
        final VarOptItemsSketch<T> vis = VarOptItemsSketch.heapify(mem, serDe);
        mergeInto(vis);
      }
    }
docs: Union the given Memory image of the sketch.
  
  <p>This method can be repeatedly called.</p>
  
  @param mem Memory image of sketch to be merged
  @param serDe An instance of ArrayOfItemsSerDe