Dave Wyland, ../../ (1978)
“ Since the FORTH definitions are nested, the throughput gain of new instructions can be achieved by modifying a small amount of code, and the remaining structures remain unchanged. All existing application programs, translators, etc. are immediately improved without change! Note that this process is reversable. Programs created for an existing system which has an extensive instruction set can be run on a simpler machine by converting the appropriate code based definitions to nested (colon) definitions. The process is also incremental: new instructions can be added one-at-a-time as desired. ”
