code: def test_double_integral(self):
      # 8) Double Integral test
      def simpfunc(y, x):  # Note order of arguments.
          return x + y
  
      a, b = 1.0, 2.0
      assert_quad(
          dblquad(simpfunc, a, b, lambda x: x, lambda x: 2 * x),
          5 / 6.0 * (b**3.0 - a**3.0),
      )
  
imports: import sys
  import math
  import numpy
  from numpy import sqrt, cos, sin, arctan, exp, log, pi
  from numpy.testing import assert_, assert_allclose, assert_array_less, assert_almost_equal
  import pytest
  from scipy.integrate import quad, dblquad, tplquad, nquad
  from scipy.special import erf, erfc
  from scipy._lib._ccallback import LowLevelCallable
  import ctypes
  import ctypes.util
  from scipy._lib._ccallback_c import sine_ctypes
  import scipy.integrate._test_multivariate
label_desc: Use image node_id 13 for calling the TestQuad obj's underlying member method code with example usage: obj.test_double_integral() and returns: unknown