Looking at my code I realize it is inconsistent with respect to underscores, but that was because I did testing on both official and Anarki, and forgot to check the code I pasted into the submission.
I'm not sure if that has anything to do with your comment about three underscores or not. (I'd appreciate an explanation either way.)
I hadn't read very close your code, so obviously I'm wrong.
Anarki's FFI is based on mzscheme's (of course), which itself imports symbols beginning with an underscore (_int, _pointer, _string, ...) These can clash with Arc's names (and they actually do, the string function for example). To overcome this issue, I added an underscore to Arc's names in Anarki (Arc's string is now mzscheme's __string).
Reading the original ac-global-name code and the corrected one I saw yours had one more underscore. I thought it was part of the correction, but it seems you just give arc2's original ac-global-name and Anarki's corrected code. Hence the mistake. Sorry for that noise :)
