I implemented the Remove button first, when `#destroy` was the obvious choice. As the controller evolved, when Move arrived, it felt (and this surprised me, but I gave it plenty of thought at the time) as though `#destroy` was still the most appropriate name as it always destroys a membership, irrespective of whether you make a new one. If it were `#update` then it would never update the model that the controller implies it should; it'd only ever create or delete 'em. Not ideal, I agree, but I'm not unhappy with this.

I don't really like jumping through the hoops required to add a new action unless I need an extra one, and then I usually find myself thinking I should have had a new controller instead. If one of the standard REST methods isn't appropriate, perhaps the controller has the wrong name? Perhaps what we should be doing here is `create`-ing something, but that the thing we're creating isn't a `DocumentSeriesGroupMembership` instance, but a virtual concept. That might not be so obvious to the reader either though…