old_contents: import unittest
  import numpy as np
  
  from sprocket.util.delta import delta
  
  class DeltaTest(unittest.TestCase):
  
      def test_delta(self):
          T = 100
          dim = 2
          data_1d = np.arange(T * dim)
          delta_1d = delta(data_1d)
  
          data_2d = data_1d.reshape(T, dim)
          delta_12 = delta(data_2d)
  
new_contents: import unittest
  import numpy as np
  
  from sprocket.util.delta import delta
  
  class DeltaTest(unittest.TestCase):
  
      def test_delta(self):
          T = 100
          dim = 2
          data_1d = np.arange(T * dim)
          delta_1d = delta(data_1d)
          assert len(delta_1d) == len(data_1d)
  
          data_2d = data_1d.reshape(T, dim)
          delta_2d = delta(data_2d)
          assert data_2d.shape == delta_2d.shape
  
subject: modify test code for delta