def uniq_stable(elems):
    """uniq_stable(elems) -> list

    Return from an iterable, a list of all the unique elements in the input,
    but maintaining the order in which they first appear.

    A naive solution to this problem which just makes a dictionary with the
    elements as keys fails to respect the stability condition, since
    dictionaries are unsorted by nature.

    Note: All elements in the input must be hashable.
    """
    unique = []
    unique_set = set()
    for nn in elems:
        if nn not in unique_set:
            unique.append(nn)
            unique_set.add(nn)
    return unique