def unique_by_index(sequence):
    """ unique elements in `sequence` in the order in which they occur

    Parameters
    ----------
    sequence : iterable

    Returns
    -------
    uniques : list
        unique elements of sequence, ordered by the order in which the element
        occurs in `sequence`
    """
    uniques = []
    for element in sequence:
        if element not in uniques:
            uniques.append(element)
    return uniques
