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Scheme In One Defun (or Scheme In One Day) is a small-footprint implementation of the Scheme programming language, written in C and designed to be embedded inside C programs. It was originally written by Professor George J. Carrette at Boston University as a demonstration in a laboratory.
- SIOD implements the original version of Scheme from the Lambda Papers, but it does not implement any of the modern language standards.
- SIOD represents a very early use of conservative garbage collection in a Lisp interpreter, a technique which was later copied by SCM and Guile.
- Compilation is implemented by emitting a fixed machine code prologue followed by a fast-loading binary representation of the parse tree to be interpreted.
- GIMP used SIOD as its primary extension language, Script-Fu, until version 2.4 was released.
- SIAG (Scheme in a grid) is a spreadsheet application using SIOD as a base.
- Festival Speech Synthesis System uses SIOD as its underlying command interpreter. 
- ^ "GIMP - Script-Fu Migration Guide". gimp.org. Retrieved November 12, 2011.
- ^ "CSTR Festival Speech Synthesis System". Retrieved May 26, 2013.