Extensible programming is a term used in computer science to describe a style of computer programming that focuses on mechanisms to extend the programming language, compiler and runtime environment. Extensible programming languages, supporting this style of programming, were an active area of work in the 1960s, but the movement was marginalized in the 1970s. Extensible programming has become a topic of renewed interest in the 21st century. Historical movement The first paper usually associated with the extensible programming language movement is M. Douglas McIlroy's 1960 paper on macros for higher-level programming languages. Another early description of the principle of extensibility occurs in Brooker and Morris's 1960 paper on the Compiler-Compiler. The peak of the movement was marked by two academic symposia, in 1969 and 1971. By 1975, a survey article on the movement by Thomas A. Standish was essentially a post mortem. The Forth programming language was an exception, but it went essentially unnoticed. Character of the historical movement As typically envisioned, an extensible programming language consisted of a base language providing elementary computing facilities, and a meta-language capable of modifying the base language. A program then consisted of meta-language modifications and code in the modified base language. The most prominent language-extension technique used in the movement was macro definition. Grammar modification was also closely associated with the movement, resulting in the eventual development of adaptive grammar formalisms. The Lisp language community remained separate from the extensible language community, apparently because, as one researcher observed, any programming language in which programs and data are essentially interchangeable can be regarded as an extendible [sic] language. ... this can be seen very easily from the fact that Lisp has been used as an extendible language for years. At the 1969 conference, Simula was presented as an extensible programming language. Standish described three classes of language extension, which he called paraphrase, orthophrase, and metaphrase (otherwise paraphrase and metaphrase being translation terms). Paraphrase defines a facility by showing how to exchange it for something previously defined (or to be defined). As examples, he mentions macro definitions, ordinary procedure definitions, grammatical extensions, data definitions, operator definitions, and control structure extensions. Orthophrase adds features to a language that could not be achieved using the base language, such as adding an i/o system to a base language that previously had no i/o primitives. Extensions must be understood as orthophrase relative to some given base language, since a