def add_unique(list,elt):
    """Add an element uniquely to a list
    """
    # Since we can't use set(), which uses memory addresses as hashes
    # for any creation or order-sensitive operation in models
    # instead we use lists and rather than foo = set(); foo.add(x)
    # we use foo = []; add_unique(foo,x)
    if elt not in list:
        list.append(elt)
    return list