pkg_3``. Similarly, ``C`` is only needed for
    ``pkg_2``, and ``D`` is only needed for ``pkg_3``::

        >>> unique_to_each({'A', 'B'}, {'B', 'C'}, {'B', 'D'})
        [['A'], ['C'], ['D']]

    If there are duplicates in one input iterable that aren't in the others
    they will be duplicated in the output. Input order is preserved::

        >>> unique_to_each("mississippi", "missouri")
        [['p', 'p'], ['o', 'u', 'r']]

    It is assumed that the elements of each iterable are hashable.

    c