áYields elements in order, ignoring serial duplicates

    >>> list(unique_justseen('AAAABBBCCDAABBB'))
    ['A', 'B', 'C', 'D', 'A', 'B']
    >>> list(unique_justseen('ABBCcAD', str.lower))
    ['A', 'B', 'C', 'A', 'D']

    r‰