code: def _detect(env):
      
      global prefer_xsltproc
      
      if env.get('DOCBOOK_PREFER_XSLTPROC',''):
          prefer_xsltproc = True
          
      if ((not has_libxml2 and not has_lxml) or (prefer_xsltproc)):
          # Try to find the XSLT processors
          __detect_cl_tool(env, 'DOCBOOK_XSLTPROC', xsltproc_com, xsltproc_com_priority)
          __detect_cl_tool(env, 'DOCBOOK_XMLLINT', xmllint_com)
  
      __detect_cl_tool(env, 'DOCBOOK_FOP', fop_com, ['fop','xep','jw'])
docs: Detect all the command line tools that we might need for creating
      the requested output formats.