A Scheme implementation that is partially compliant with both R4RS and R5RS . To be more specific RScheme implements all of R4RS except transcript-on and transcript-off. RScheme also implements all of R5RS except define-syntax .
RScheme is currently also the only known public available scheme implementation with a realtime safe garbage collector.
See http://www.rscheme.org for details. An overview of RScheme is available at http://www.rscheme.org/rs/a/2005/overview/