Ah, I should have said that I don't think we can merge this with ```only```. In the ```one``` case, the value that represents 'no change' in a two-way binding is the one returned by ```one()```, since that is what we used going the other direction.

We could potentially parameterize this so that:

```javascript
var makeAggregatingBinder = function(fixpointMethodName){ /* ... */ };
var makeOneBinder = makeAggregatingBinder('one');
var makeOnlyBinder = makeAggregatingBinder('has');
```