def uniqify(seq):
    """`Uniqify` a sequence, preserving order.

    A plain-vanilla version of itertools' `unique_everseen`.

    Example
    -------
    >>> s = list('zyxabccabxyz')
    >>> uniqify(s)
    ['z', 'y', 'x', 'a', 'b', 'c'

    Returns
    -------
    list
    """

    if PY37:
        # Credit: Raymond Hettinger
        return list(dict.fromkeys(seq))
    else:
        # Credit: Dave Kirby
        # https://www.peterbe.com/plog/uniqifiers-benchmark
        seen = set()
        # We don't care about truth value of `not seen.add(x)`;
        # just there to add to `seen` inplace.
        return [x for x in seq if x not in seen and not seen.add(x)]