'Test the trace atom.'

def test_trace(self):

    expr = trace(self.A)
    self.assertEqual(expr.sign, s.UNKNOWN)
    self.assertEqual(expr.curvature, s.AFFINE)
    self.assertEqual(expr.size, (1, 1))
    with self.assertRaises(Exception) as cm:
        trace(self.C)
    self.assertEqual(str(cm.exception), 'Argument    to    trace    must    be    a    square    matrix.')
