While this use of `@extend` is an interesting feature to consider across compass more broadly, it requires a bit more thought. I'd prefer to discuss it as a separate feature, with an issue/pull-request of its own. As far as I'm aware, Compass currently has no output (including placeholder classes) unless explicitly requested — and I'd like to keep it that way. It's not hard for a user to create their own clearfix classes/placeholders as they want, and if we plan to provide that feature, it should be opt-in.