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export class PeekIterator<T> implements IterableIterator<T> {  private _peeked: IteratorResult<T> | undefined = undefined;  private readonly _iter: Iterator<T>;
  constructor(iter: Iterator<T>) {    this._iter = iter;  }
  [Symbol.iterator](): IterableIterator<T> {    return this;  }
  next(): IteratorResult<T> {    if (this._peeked !== undefined) {      const p = this._peeked;      this._peeked = undefined;      return p;    }    return this._iter.next();  }
  peek(): IteratorResult<T> {    if (this._peeked !== undefined) {      return this._peeked;    }    return (this._peeked = this._iter.next());  }}