def test_negativeindex():
    from ase.atoms import Atoms
    from ase.constraints import FixScaled

    a1 = Atoms(symbols = 'X2',
               positions = [[0.,0.,0.], [2.,0.,0.], ],
               cell = [[4.,0.,0.], [0.,4.,0.], [0.,0.,4.], ],
               )

    fs1 = FixScaled(a1.get_cell(), -1, mask=(True, False, False))
    fs2 = FixScaled(a1.get_cell(), 1, mask=(False, True, False))

    a1.set_constraint([fs1,fs2])

    # reassigning using atoms.__getitem__
    a2 = a1[0:2]

    assert len(a1._constraints) == len(a2._constraints)