feature not defined in terms of the base language must be defined in terms of some other language. Orthophrase corresponds to the modern notion of plug-ins. Metaphrase modifies the interpretation rules used for pre-existing expressions. It corresponds to the modern notion of reflection. Death of the historical movement Standish attributed the failure of the extensibility movement to the difficulty of programming successive extensions. An ordinary programmer might build a single shell of macros around a base language, but if a second shell of macros was to be built around that, the programmer would have to be intimately familiar with both the base language and the first shell; a third shell would require familiarity with the base and both the first and second shells; and so on. (Note that shielding the programmer from lower-level details is the intent of the abstraction movement that supplanted the extensibility movement.) Despite the earlier presentation of Simula as extensible, by 1975, Standish's survey does not seem in practice to have included the newer abstraction-based technologies (though he used a very general definition of extensibility that technically could have included them). A 1978 history of programming abstraction from the invention of the computer to the (then) present day made no mention of macros, and gave no hint that the extensible languages movement had ever occurred. Macros were tentatively admitted into the abstraction movement by the late 1980s (perhaps due to the advent of hygienic macros), by being granted the pseudonym syntactic abstractions. Modern movement In the modern sense, a system that supports extensible programming will provide all of the features described below. Extensible syntax This simply means that the source language(s) to be compiled must not be closed, fixed, or static. It must be possible to add new keywords, concepts, and structures to the source language(s). Languages which allow the addition of constructs with user defined syntax include Racket, Camlp4, OpenC++, Seed7, Red, Rebol, and Felix. While it is acceptable for some fundamental and intrinsic language features to be immutable, the system must not rely solely on those language features. It must be possible to add new ones. Extensible compiler In extensible programming, a compiler is not a monolithic program that converts source code input into binary executable output. The compiler itself must be extensible to the point that it is really a collection of plugins that assist with the translation of source language input into