I agree, the naming is odd. `onChange` is almost always the name of a function that is called by a user event and passed an Event object, not called by the programmer and passed a raw value. Is it supposed to be used as the first line in a form onChange handler? Perhaps something like

    onChange: function(e) { this.cursor.onChange(e.value);}

Even in that case, I think it is a lot more intuitive to use a function name like `change`, `mutate`, or `transact`. Personally, `onChange` is something I want to assign, not something I want to call. Additionally, it doesn't look like it can be passed a raw Event object, so you can't shorthand it in a render method and avoid creating an onChange callback.