import { PipelineOperator } from "../../core";
/**
 * Combines two aggregations into a single result set.
 *
 * See {@link https://www.mongodb.com/docs/manual/reference/operator/aggregation/unionWith usage}.
 *
 * @param collection
 * @param expr
 * @param opt
 */
export declare const $unionWith: PipelineOperator;