docs: function footnoteTokensToMeta(tokens, md) {
    return tokens
      .map((token, i) => { return token.type === 'footnote_open' ? i : -1; })
      .filter(i => i >= 0)
      .map((i) => {
        const inlineToken = tokens[i + 2];
        assert(inlineToken.type === 'inline');
        const anchorToken = tokens[i + 3];
        assert(anchorToken.type === 'footnote_anchor');
  
        const content = md.renderer.render([inlineToken], md.options, {});
        const id = anchorToken.meta.id + 1;
        const meta = Object.assign({}, anchorToken.meta, { id });
        return Object.assign({}, { content }, meta);
      });
  }