SDoP (Simple DocBook Processor) reads a Simplified DocBook XML file, processes it into typeset pages, and outputs the result as PostScript (which can easily be converted to a PDF). It is "simple" because it supports only a subset of DocBook, and also because it does not make use of a DTD or stylesheets or any other heavyweight apparatus. It is a single program. SDoP is used to format the Exim reference manual.
