code: def test_boundary_difference_linear_2D(self):
      self.__test_boundary_term_2d(
          boundary_difference_linear, (self.image, False)
      )
  
imports: import unittest
  import numpy
  from numpy.testing import assert_array_equal
  from medpy.graphcut import graph_from_voxels
  from medpy.graphcut.energy_voxel import boundary_difference_linear, boundary_difference_exponential, boundary_difference_division, boundary_difference_power, boundary_maximum_linear, boundary_maximum_exponential, boundary_maximum_division, boundary_maximum_power, regional_probability_map
label_desc: Use image node_id 1 for calling the TestEnergyVoxel obj's underlying member method code with example usage: obj.test_boundary_difference_linear_2D() without return types