import { PipelineOperator } from "../../core";
/**
 * Separates documents into groups according to a "group key" and output one document for each unique group key.
 *
 * See {@link https://www.mongodb.com/docs/manual/reference/operator/aggregation/group usage}.
 *
 * @param collection
 * @param expr
 * @param options
 * @returns
 */
export declare const $group: PipelineOperator;