import type { Extended } from './isExtension.js';
export declare const $SUM: unique symbol;
export type $SUM = typeof $SUM;
export type SUM<A, B> = Extended<{
    [$SUM]: [A, B];
}>;
export declare const $sum: <A, B>(a: A, b: B) => SUM<A, B>;
export declare const isSum: (input: unknown) => input is {
    [$SUM]: unknown;
};