import type { ParserSymbol } from '../models.js';
/**
 * Checks it the given symbol is defined. If it's not, it won't throw an error,
 * but instead it will tell us to backtrack, and we will continue with the next symbol
 * in the production rule.
 *
 * @param symbol - The grammar symbol that may match or not
 *
 * @returns The grammar symbol
 */
export declare function optional(symbol: ParserSymbol): ParserSymbol;
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