SGML documents are processed using a suite of free tools produced by James Clark.
Jade is a system which implements the DSSSL language for document formatting and, in the present case, transformation. The difference is that `formatting' refers to the process of turning an SGML document into a printed, or otherwise readable, form, and `transformation' refers to the processing of an SGML document into another SGML document. Thus the conversion to HTML is really `transformation', but since HTML is not much cop as a high-level markup language, the HTML transformation is much more akin to a formatting step; likewise the `formatting' into LaTeX. In fact, DSSSL has a transformation language as well as a formatting language; Jade implements only the formatting language, but with extensions which allow it to be used very easily for transformation.
Along with Jade is the SP toolkit. This consists of a standalone parser,
nsgmls, and various markup editing tools.