def occurs_once_in_sets(set_sequence):
    """Returns the elements that occur only once in the sequence of sets set_sequence.
    The elements are returned as a set."""
    occuronce = set()
    deleted = set()
    for setx in set_sequence:
        for sety in setx:
            if (sety in occuronce):
                deleted.add(sety)
                occuronce.remove(sety)
            elif (sety not in deleted):
                occuronce.add(sety)
    return occuronce