Ypsilon Scheme is a free software

Free software

Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions that only ensure that further recipients can also do...

 implementation of the R6RS standard of Scheme. It implements mostly concurrent garbage collection, which is optimized for multi-core CPU systems. Its author, Yoshikatsu Fujita, developed the Ypsilon implementation to satisfy the need for a real-time Scheme implementation suitable for computer game development without giving up a garbage collected dynamic interpreter.