code: func hasOneUsefulIterator(its []graph.Iterator) graph.Iterator {
  	usefulCount := 0
  	var usefulIt graph.Iterator
  	for _, it := range its {
  		switch it.Type() {
  		case graph.All:
  			continue
  		case graph.Optional:
  			// Optional is weird -- it's not useful, but we can't optimize
  			// away from it. Therefore, we skip this optimization
  			// if we see one.
  			return nil
  		default:
  			usefulCount++
  			usefulIt = it
  		}
  	}
  
  	if usefulCount == 1 {
  		return usefulIt
  	}
  	if usefulCount == 0 {
  		// It's full of All iterators. We can safely return one of them.
  		return its[0]
  	}
  	return nil
  }
docs: // There are two "not-useful" iterators -- namely graph.Null which returns
  // nothing, and graph.All which returns everything. Particularly, we want
  // to see if we're intersecting with a bunch of graph.All iterators, and,
  // if we are, then we have only one useful iterator.
  //
  // We already checked for hasAnyNullIteratators() -- so now we're considering
  // All iterators.