import { Iterator } from "../../lazy";
import { AnyObject, Options } from "../../types";
interface InputExpr {
    readonly coll: string | AnyObject[];
    readonly pipeline?: AnyObject[];
}
/**
 * Combines two aggregations into a single result set.
 *
 * See {@link https://www.mongodb.com/docs/manual/reference/operator/aggregation/unionWith usage}.
 */
export declare function $unionWith(collection: Iterator, expr: InputExpr | string | AnyObject[], options: Options): Iterator;
export {};