Yeah, definitely. The reason for the bug is because the Arc compiler uses Racket's "append" to do the join, which means it first needs to convert the Arc list to a Racket list, do the append, then convert back to an Arc list.
So there's two ways to fix this:
1) Change Arc to be like Arc/Nu, so that it doesn't need to do the conversion. I don't expect this to be too hard.
2) Write + in Arc, and have it call "join" when the first argument is a list. Then there's no discrepency between the two.
I actually dislike how the overloaded + is written in Arc, so I think having it call join instead would be great.
