To elaborate: Scheme and Arc aren't that different to implement. Parsing & literals are largely the same (Arc doesn't use booleans, vectors, et al.), Lisp constructs are fairly standard (quote, if, assign, fn, etc.), and Arc even borrows high-level features like TCO and continuations. Scheme uses hygienic macros, which seem harder to implement than Arc/CL-style macros anyway. Moreover, Arc is simple: the "primitive" things are in ac.scm and the "meat" is in arc.arc. Once you get the Scheme side working, you get the rest of Arc for free.