declare global {
    interface Set<T> {
        union?<U>(other: Set<U>): Set<T | U>;
        difference?<U>(other: Set<U>): Set<T>;
    }
}
export declare function union<T, U>(setA: Set<T>, setB: Set<U>): Set<T | U>;
export declare function difference<T, U extends T>(setA: Set<T>, setB: Set<U>): Set<T>;