TXL is a unique programming language specifically designed to support computer software analysis and source transformation tasks. It is the evolving result of more than fifteen years of concentrated research on rule-based structural transformation as a paradigm for the rapid solution of complex computing problems. The TXL programming language is a hybrid functional / rule-based language with unification, implied iteration and deep pattern match. Each TXL program has two components: * A Description of the Structures to be Transformed Specified as an EBNF grammar, in context-free ambiguous form. * A Set of Structural Transformation Rules Specified by example, using pattern/replacement pairs.