anything. For example, an extensible compiler will support the generation of object code, code documentation, re-formatted source code, or any other desired output. The architecture of the compiler must permit its users to "get inside" the compilation process and provide alternative processing tasks at every reasonable step in the compilation process. For just the task of translating source code into something that can be executed on a computer, an extensible compiler should: use a plug-in or component architecture for nearly every aspect of its function determine which language or language variant is being compiled and locate the appropriate plug-in to recognize and validate that language use formal language specifications to syntactically and structurally validate arbitrary source languages assist with the semantic validation of arbitrary source languages by invoking an appropriate validation plug-in allow users to select from different kinds of code generators so that the resulting executable can be targeted for different processors, operating systems, virtual machines, or other execution environment. provide facilities for error generation and extensions to it allow new kinds of nodes in the abstract syntax tree (AST), allow new values in nodes of the AST, allow new kinds of edges between nodes, support the transformation of the input AST, or portions thereof, by some external "pass" support the translation of the input AST, or portions thereof, into another form by some external "pass" assist with the flow of information between internal and external passes as they both transform and translate the AST into new ASTs or other representations Extensible runtime At runtime, extensible programming systems must permit languages to extend the set of operations that it permits. For example, if the system uses a byte-code interpreter, it must allow new byte-code values to be defined. As with extensible syntax, it is acceptable for there to be some (smallish) set of fundamental or intrinsic operations that are immutable. However, it must be possible to overload or augment those intrinsic operations so that new or additional behavior can be supported. Content separated from form Extensible programming systems should regard programs as data to be processed. Those programs should be completely devoid of any kind of formatting information. The visual display and editing of programs to users should be a translation function, supported by the extensible compiler, that translates the program data into forms more suitable for viewing or editing. Naturally, this should be a two-way translation. This is