def identical(s1, s2):
    """
    Verify that the nodes in {s1} and {s2} are identical. This has to be done carefully since
    we must avoid triggering __eq__
    """
    # for the pairs
    for n1, n2 in zip(s1, s2):
        # check them for _identity_, not _equality_
        if n1 is not n2: return False
    # all done
    return True