Courses and course formats

/**
     * Course-specific information to be output on any course page (usually above navigation bar)
     *
     * Example of usage:
     * define
     * class format_FORMATNAME_XXX implements renderable {}
     *
     * create format renderer in course/format/FORMATNAME/renderer.php, define rendering function:
     * class format_FORMATNAME_renderer extends plugin_renderer_base {
     *     protected function render_format_FORMATNAME_XXX(format_FORMATNAME_XXX $xxx) {
     *         return html_writer::tag('div', 'This is my header/footer');
     *     }
     * }
     *
     * Return instance of format_FORMATNAME_XXX in this function, the appropriate method from
     * plugin renderer will be called
     *
     * @return null|renderable null for no output or object with data for plugin renderer
     */
    public function course_header() {
        return null;
    }