"But the second part is bad, I think: the elements inside l should stay the same."
I agree. I really think this is a simple coding mistake in ac.scm. I talk more about this here, a reply to that thread you linked to: http://arclanguage.org/item?id=17114
I completely forgot about this difference between + and 'join after that thread, and I don't remember encountering it before that either. How fitting it's a testing framework that brings this bug to the surface! :-p
