lower))
        ['A', 'B', 'C', 'D']

    Sequences with a mix of hashable and unhashable items can be used.
    The function will be slower (i.e., `O(n^2)`) for unhashable items.

    Remember that ``list`` objects are unhashable - you can use the *key*
    parameter to transform the list to a tuple (which is hashable) to
    avoid a slowdown.

        >>> iterable = ([1, 2], [2, 3], [1, 2])
        >>> list(unique_everseen(iterable))  # Slow
        [[1, 2], [2, 3]]
        >>> list(unique_everseen(iterable, key=tuple))  # Faster
        [[1, 2], [2, 3]]

    Similary, you may want to convert unhashable ``set`` objects with
    ``key=frozenset``. For ``dict`` objects,
    ``key=lambda x: frozenset(x.items())`` can be used.

    N)