code: @Override
  	public Iterator<E> iterator() {
  		combine();
  		if(combined==null) {
  			Set<E> emptySet = java.util.Collections.emptySet();
  			return emptySet.iterator();
  		}
  		return combined.iterator();
  	}
docs: Triggers combining.
  
  TODO: Iterate without combining - benchmark speed versus complexity