            # not always true but usually true
            r0 = neutron0.state.position
            self.assertTrue(r[0]!=r0[0])
            self.assertTrue(r[1]!=r0[1])
            # always true
            self.assertTrue(abs(r[0]-r0[0])<=dx/2)
            self.assertTrue(abs(r[1]-r0[1])<=dy/2)
            self.assertTrue(abs(r[2])==r0[2])

            self.assertEqual(
                tuple(n.state.velocity),