I see your point @novemberborn, but I'm also not too concerned about specifying.  I think people will tend to call it like `promise.then(...)` since the name "then" lends itself to being very readable that way, and there are (obviously) strong use cases for passing around the promise, but the use cases for simply passing around a `then` method aren't as clear (even though that'd work just fine with some implementations)

We could clarify with a note suggesting that `then` be called with it's associated promise as the `thisArg`, but I don't feel strongly that we need to.