import type { PropExpression } from "../../models";
/**
 * Checks whether a given propositional expression is a well-formed formula (WFF).
 *
 * A WFF follows these rules:
 * 1) Any single variable itself is a WFF.
 * 2) Any WFF can be prefixed with "~" to form another WFF.
 * 3) Any two WFFs can be combined with "&", "|", "=>", or "<=>" inside parentheses to form another WFF.
 *
 * Parentheses are mandatory when joining two WFFs using binary operators.
 *
 * @param expression - The propositional expression as an array of PropSymbols.
 * @returns `true` if the expression is a valid WFF, otherwise `false`.
 * @category Validators
 */
export declare function isWellFormedFormula(expression: PropExpression): boolean;