/**
 * String Expression Operators: https://docs.mongodb.com/manual/reference/operator/aggregation/#string-expression-operators
 */
import { ExpressionOperator } from "../../../core";
/**
 * Converts a string to lowercase.
 *
 * @param obj
 * @param expr
 * @returns {string}
 */
export declare const $toLower: ExpressionOperator;
