That's what I was worried about ; ) This probably needs more thinking and discussion...

By making the arguments optional it is impossible to style any note in the preface (since `@part` is the 2nd arg)

(which is why I was suggesting making them explicit and adding an `any` keyword for `@kind` and `@part`)

By using both, optional args and `any` it creates multiple ways to call a mixin (`.mixin()`, `.mixin(any; any)`, `.mixin(any)`)

Hmm, maybe http://lesscss.org/features/#mixins-parametric-feature-named-parameters are another solution to this problem (and may even allow partial-evaluation of mixins??)