Abstract: A method of extending a hierarchial programming language that retains compatibility with programs written for the original language. This method also allows a language to be defined that permits common commands to be abbreviated into shorter forms, providing a more user friendly system, while still permitting the longer form of a command to perform the intended way. The method also allows such a language to be restructured and extended by inserting a new level into the parse tree of the language. The method comprises inserting a default node into the parse tree of the language at the point where the extension, or the shortened command form, occurs. When the disclosed language parser method interprets the tree, the default node will be treated as if the mnemonic of the default node were actually in the command being parsed at the position where the default node appears in the tree. The disclosed method of processing the new parse tree also allows multiple commands per line, as required by existing standards.