import { Downstream, Upstream } from './types';
export declare function subDownstream<T, K extends keyof T>(src: Downstream<T>, key: K, current: () => T[K] | undefined): Downstream<T[K] | undefined>;
export declare function subUpstream<T, K extends keyof T>(src: Upstream<T>, key: K, ref: () => T | undefined): Upstream<T[K]>;