Since the spec already says `thenable.then`, perhaps you could clarify with a note?

    The `then` method should be called on the `thenable`, that is:
    
    ```js
    thenable.then(resolvePromise, rejectPromise);
    ```
    
    Not:
    
    ```js
    var then = thenable.then;
    then(resolvePromise, rejectPromise);
    ```