code: function(model, options) {
        options || (options = {});
        model = this.getByCid(model) || this.get(model);
        if (!model) return null;
        delete this._byId[model.id];
        delete this._byCid[model.cid];
        this.models.splice(this.indexOf(model), 1);
        this.length--;
        if (!options.silent) model.trigger('remove', model, this, options);
        this._removeReference(model);
        return model;
      }
docs: Internal implementation of removing a single model from the set, updating hash indexes for `id` and `cid` lookups.