code: func (it *Not) Contains(ctx context.Context, val graph.Value) bool {
  	graph.ContainsLogIn(it, val)
  	it.runstats.Contains += 1
  
  	if it.primaryIt.Contains(ctx, val) {
  		return graph.ContainsLogOut(it, val, false)
  	}
  
  	it.err = it.primaryIt.Err()
  	if it.err != nil {
  		// Explicitly return 'false', since an error occurred.
  		return false
  	}
  
  	it.result = val
  	return graph.ContainsLogOut(it, val, true)
  }
docs: // Contains checks whether the passed value is part of the primary iterator's
  // complement. For a valid value, it updates the Result returned by the iterator
  // to the value itself.