code: func (it *Fixed) Optimize() (graph.Iterator, bool) {
  	if len(it.values) == 1 && it.values[0] == nil {
  		return NewNull(), true
  	}
  
  	return it, false
  }
docs: // Optimize() for a Fixed iterator is simple. Returns a Null iterator if it's empty
  // (so that other iterators upstream can treat this as null) or there is no
  // optimization.