import { PipelineOperator } from "../../core";
/**
 * Performs a left outer join to another collection to filter in documents from the "joined" collection for processing.
 *
 * See {@link https://www.mongodb.com/docs/manual/reference/operator/aggregation/lookup/ usage}
 *
 * @param collection
 * @param expr
 * @param options
 */
export declare const $lookup: PipelineOperator;