The package preprocesses input files to a Lua(La)TeX run, on the fly. The user defines Lua regular expressions to search for patterns and modify input lines (or entire paragraphs) accordingly, before TeX reads the material. In this way, documents may be prepared in a non-TeX language (e.g., some lightweight markup language) and turned into ‘proper’ TeX for processing. The source of the documentation is typed in such a lightweight language and is thus easily readable in a text editor (the PDF file is also available, of course); the transformation to TeX syntax via Interpreter's functions is explained in the documentation itself. Interpreter is implemented using the author’s gates (lua version), and works for plain TeX and LaTeX, but not ConTeXt.