import { Iterator } from "../../lazy";
import { Any, AnyObject, Options } from "../../types";
declare const ID_KEY = "_id";
interface InputExpr extends AnyObject {
    [ID_KEY]: Any;
}
/**
 * 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}.
 */
export declare function $group(coll: Iterator, expr: InputExpr, options: Options): Iterator;
export {};