def footnote_ref(self, key, index):
html = (<EOL>'<STR_LIT>'<EOL>'<STR_LIT>'<EOL>) % (escape(key), escape(key), index)<EOL>return html<EOL>
Rendering the ref anchor of a footnote.

        :param key: identity key for the footnote.
        :param index: the index count of current footnote.