Most of SCM.
Arrays, gsubrs, compiled closures, records, Ecache, syntax-rules
macros, and safeports.
Real and Complex functions. Fast mixed type arithmetics.
Syntax checking and memoization of special forms by evaluator. Storage allocation strategy and parameters.
Siod, written by George Carrette, was the starting point for SCM. The major innovations taken from Siod are the evaluator’s use of the C-stack and being able to garbage collect off the C-stack (see Garbage Collection).
There are many other contributors to SCM. They are acknowledged in the file ChangeLog, a log of changes that have been made to scm.