code: def test_mult_aff():
      r"""Test matrix multiplication using None as identity"""
      A = np.array([[1.0, 2.0], [3.0, 4.0]])
      B = np.array([[2.0, 0.0], [0.0, 2.0]])
  
      C = imwarp.mult_aff(A, B)
      expected_mult = np.array([[2.0, 4.0], [6.0, 8.0]])
      assert_array_almost_equal(C, expected_mult)
  
      C = imwarp.mult_aff(A, None)
      assert_array_almost_equal(C, A)
  
      C = imwarp.mult_aff(None, B)
      assert_array_almost_equal(C, B)
  
      C = imwarp.mult_aff(None, None)
      assert_equal(C, None)
  
imports: import numpy
  import nibabel.eulerangles
  from numpy.testing import assert_equal, assert_array_equal, assert_array_almost_equal, assert_raises
  from dipy.core.interpolation import interpolate_scalar_2d, interpolate_scalar_3d
  from dipy.data import get_fnames
  from dipy.align import floating
  from dipy.align import imwarp
  from dipy.align import metrics
  from dipy.align import vector_fields
  from dipy.align import VerbosityLevels
  from dipy.align.imwarp import DiffeomorphicMap
  from dipy.tracking.streamline import deform_streamlines
  from dipy.testing.decorators import set_random_number_generator
label_desc: Use image node_id 1 for calling a global function with example usage: test_mult_aff() without return types