simple, modular, and lightweight Scheme implementation

 Scheme48 is an implementation of Scheme written by Richard Kelsey
 and Jonathan Rees. It is based on a byte-code interpreter and
 incorporates such elements as a sophisticated module system, a
 pre-emptive & user-level thread system, a well-integrated
 interactive environment, Clinger's explicit renaming low-level
 macros, and decent efficiency for its architecture.