code: def testAdditiveSparseRaises(self):
      # cant add different approximations
      with pm.Model() as model:
          cov_func = pm.gp.cov.ExpQuad(3, [0.1, 0.2, 0.3])
          gp1 = pm.gp.MarginalApprox(cov_func=cov_func, approx="DTC")
          gp2 = pm.gp.MarginalApprox(cov_func=cov_func, approx="FITC")
          with pytest.raises(Exception) as e_info:
              gp1 + gp2
  
imports: from functools import reduce
  from operator import add
  import numpy
  import numpy.testing
  import pytest
  import pymc
  from pymc.math import cartesian
label_desc: Use image node_id 5 for calling the TestGPAdditive obj's underlying member method code with example usage: obj.testAdditiveSparseRaises() without return types