or, which may require significant
    storage.

    To check whether a peekable is exhausted, check its truth value:

        >>> p = peekable(['a', 'b'])
        >>> if p:  # peekable has items
        ...     list(p)
        ['a', 'b']
        >>> if not p:  # peekable is exhausted
        ...     list(p)
        []

    c