def render(self, mark_lib=True):
        """Render a single frame in a traceback."""
        return FRAME_HTML % {
            "id": self.id,
            "filename": escape(self.filename),
            "lineno": self.lineno,
            "function_name": escape(self.function_name),
            "lines": self.render_line_context(),
            "library": "library" if mark_lib and self.is_library else "",
        }