all the elements of *iterable* are unique (no two
elements are equal).

    >>> all_unique('ABCB')
    False

If a *key* function is specified, it will be used to make comparisons.

    >>> all_unique('ABCb')
    True
    >>> all_unique('ABCb', str.lower)
    False

The function returns as soon as the first non-unique element is
encountered. Iterables with a mix of hashable and unhashable items can
be used, but the function will be slower for unhashable items.
FT)