code: function (source, opts, cb) {
          var callback,
              options;
  
          if (arguments.length === 2) {
              options = defaults();
              callback = opts;
          } else {
              options = defaults(opts);
              callback = cb;
          }
  
          try {
              source = api.clean(source);
              callback(null, api.insert(source, getToc(source, options)));
          } catch (err) {
              callback(err, null);
          }
      }
docs: Inserts a TOC object into a source
  @param {String} source
  @param {Object} [opts]
  @param {Number} [opts.maxDepth]
  @param {Char}   [opts.bullet]
  @param {Function} cb