The official specification of the Scheme programming language is the ``R^4RS'' (William Clinger and Jonathan Rees (editors), Revised^4 Report on the Algorithmic Language Scheme, 1991). A slightly modified version of an earlier revision of this report was adopted as an IEEE an ANSI standard in 1990 (IEEEStd1178-1990, IEEE Standard for the Scheme Programming Language, 1991).
The dialect of Scheme implemented by Elk (a superset of the official language) is described in the Reference Manual for the Elk Extension Language Interpreter that is included in the Elk distribution as troff source and preformatted PostScript files. Reference manuals for the various predefined Elk extensions (such as the UNIX and X11 extensions) are also part of the distribution; see the file ``doc/README'' for an overview of the available documentation.
This manual supersedes the document Interfacing Scheme to the ``Real World'' that was included in earlier versions of Elk.
An article about Elk has appeared in USENIX Computing Systems in 1994 (Oliver Laumann and Carsten Bormann, Elk: The Extension Language Kit, USENIX Computing Systems, vol. 7, no. 4, pp. 419-449).
A recent example of an application that uses Elk as its extension language implementation is freely available in source and binary form as http://www.informatik.uni-bremen.de/~net/unroff. unroff is a programmable, extensible troff translator with Scheme-based back-ends for the Hypertext Markup Language. The source code shown in Appendix B has been directly taken from the unroff source; authors of Elk-based applications are encourage to reuse this and other parts of the unroff source for their own projects.