Meta-programming, or writing programmes that customize, guide and modify other programmes, is not a very new idea (LISP programmers have been doing it for decades) - but XML gives it a profoundly new twist. XML greatly increases the ability of the user to exercise control over computer programmes, by editing easily understandable text files. These programmes know as little as possible about what they are actually being used to do. Instead, their structure and behavior are described with XML in domain-specific languages, and the programmes interpret the descriptions.