export declare const $IS_EXTENSION: unique symbol;
export type $IS_EXTENSION = typeof $IS_EXTENSION;
export type Extended<VALUE> = {
    [$IS_EXTENSION]: true;
} & VALUE;
export type Unextended<VALUE> = {
    [$IS_EXTENSION]?: false;
} & VALUE;
export declare const isExtension: (input: unknown) => input is {
    [$IS_EXTENSION]: true;
};