def unique_append(old_list, new_list):
    """
    Add items from new_list to end of old_list if those items are not 
    already in old list -- returned list will have unique entries. 
    Preserve order (which is why we can't do this quicker with dicts).
    """
    combined = old_list
    for item in new_list:
        if item not in combined:
            combined.append(item)
    return combined