import { IterableX } from '../iterable';
import { PartialObserver } from '../observer';
/**
 * Lazily invokes observer methods for each value in the sequence, and upon successful or exceptional termination.
 * @param {Iterable<TSource>} source Source sequence.
 * @param {PartialObserver<TSource>} observer Observer to invoke notification calls on.<
 * @return {Ierable<TSource>} Sequence exhibiting the side-effects of observer method invocation upon iteration.
 */
export declare function tap<TSource>(source: Iterable<TSource>, observer: PartialObserver<TSource>): IterableX<TSource>;