def unique(A):
    """
    Returns elements of A that are unique as defined by 'is' an equivalence relation.
    """
    unique = []
    for a1 in A:
        count = 0
        for a2 in A:
            if a1 is a2:
                count +=1
                if count > 1:
                    break
        if count == 1:
            unique.append(a1)

    return unique