So what is going on here? what is with the extra tag delimiter types... is this garbage?

Extra to what? Are you pretending this is somehow related to XML? It's not, so don't.

and it does work!

Actually, I'm pretty sure it'll crap out for

(given this="...")
  <else>
     the answer is "else"!
  </else>
  <else>
     the answer was not "else"
  </else>
 (/given)
[download]

As a side note, I hope the square brackets are handled by some other layer and have nothing to do with the engine you are presenting. If they are handled by your engine, you're needlessly limiting the content to be this UBB-ish markup, and there's absolutely no reason to do so.