code: func (it *Fixed) Next(ctx context.Context) bool {
  	graph.NextLogIn(it)
  	if it.lastIndex == len(it.values) {
  		return graph.NextLogOut(it, false)
  	}
  	out := it.values[it.lastIndex]
  	it.result = out
  	it.lastIndex++
  	return graph.NextLogOut(it, true)
  }
docs: // Next advances the iterator.