code: func (it *Not) Next(ctx context.Context) bool {
  	graph.NextLogIn(it)
  	it.runstats.Next += 1
  
  	for it.allIt.Next(ctx) {
  		if curr := it.allIt.Result(); !it.primaryIt.Contains(ctx, curr) {
  			it.result = curr
  			it.runstats.ContainsNext += 1
  			return graph.NextLogOut(it, true)
  		}
  	}
  	it.err = it.allIt.Err()
  	return graph.NextLogOut(it, false)
  }
docs: // Next advances the Not iterator. It returns whether there is another valid
  // new value. It fetches the next value of the all iterator which is not
  // contained by the primary iterator.