You probably know that, but let me point out that the order of the arguments is wrong. That said, the expression shouldn't hang.
The rem function hangs because it tries to coerce the function to a cons (arc.arc:483) but coerce returns what it was given when it doesn't know what to do (ac.scm:787). So rem calls itself indefinitely. Maybe coerce should throw an error in that case? I'll try that out and let you know.
