Chicken Scheme implements R5RS Scheme using the continuation passing style underneath ("Cheney on the MTA"), so it is never strictly exposed to the programmer (

Chicken Scheme implements R5RS Scheme using the continuation passing style underneath ("Cheney on the MTA"), so it is never strictly exposed to the programmer (no syntax changes), whilst giving them the performance improvements and making call/cc macros practically free. That's what I would call first class language support.