rntz's port of the Arc compiler to Arc (http://github.com/nex3/arc/tree/arcc) copied the Arc runtime portion of the original ac.scm into a new ar.scm file.
I was playing around with changing the runtime and found it inconvenient to have to make changes to the runtime in two places, so this version avoids code duplication by having the original Arc compiler also use the extracted Arc runtime ar.scm.
