code: function(model, options) {
        options || (options = {});
        model = this._prepareModel(model, options);
        if (!model) return false;
        var already = this.getByCid(model) || this.get(model);
        if (already) throw new Error(["Can't add the same model to a set twice", already.id]);
        this._byId[model.id] = model;
        this._byCid[model.cid] = model;
        var index = options.at != null ? options.at :
                    this.comparator ? this.sortedIndex(model, this.comparator) :
                    this.length;
        this.models.splice(index, 0, model);
        model.bind('all', this._onModelEvent);
        this.length++;
        if (!options.silent) model.trigger('add', model, this, options);
        return model;
      }
docs: Internal implementation of adding a single model to the set, updating hash indexes for `id` and `cid` lookups. Returns the model, or 'false' if validation on a new model fails.