def _add_documentation_link(self, links, section, variable_name, default=None):
        """
          :param list links: List of links to append link of the form "Section: <link>", if link available
          :param str section: Name of the section (concept, command line options, API reference)
          :param str variable_name: Variable name in main module that should hold URL to documentation
          :param str default: Default URL to documentation
        """

        url = getattr(sys.modules['__main__'], variable_name, default)

        if url:
            links.append('%s: %s' % (section, url))