This is interesting. I skimmed some introductory parts of https://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-908.pdf as well as the definition of the SCAN function and searched the PDF for "reference count" to see how it does what you propose. At the moment I don't have the impression that ASAP dynamically reference counts anything. Let alone selectively reference count certain objects but use a tracing GC for others. Could you tell me where exactly to look for details? Wouldn't references from RC objects to GC objects, and vice versa, be an absolute pain to deal with?