The @mixes tag indicates that the current object mixes in all the members from
which is a @mixin.
To start, we document a mixin with the @mixin tag:
Now we add a FormButton class and call a "mix" function that mixes all of the Eventful functions into FormButton, so that FormButton can also fire events and have listeners. We use the @mixes tag to indicate that FormButton mixes the Eventful functions.