export type Rule<Output> = (input: unknown) => Output;
export type Static<Output> = Output extends Rule<infer V> ? V : Output;
export type Key = string | number;
export type Constrained<ConstraintSymbol extends symbol, Type> = Type & {
    readonly _constraintSymbol: ConstraintSymbol;
};
export declare function constrain<ConstraintSymbol extends symbol, T>(symbol: ConstraintSymbol, input: T): Constrained<typeof symbol, T>;
export declare function guard<R extends Rule<unknown>>(rule: R): (input: unknown) => input is Static<R>;