Basically, I think it's a bad idea to change `ac` into something which sometimes compiles Arc code and sometimes does something more like code-walking over s-expressions. I think Anarki's existing "$ ... unquote" syntax already serves this purpose and uses the same kind of code-walking but does so with a better separation of concerns.
Moreover, the way you're taking out the |foo| syntax so you can redefine it to be a variant of $ seems like a net loss.
(Some of my other comments on that review are less fundamental objections: Style nitpicks, observations of bugs, or wild ideas that I don't really expect anyone to act on in the short term.)
