code: def testValidateDFPositive(self):
      with self.assertRaisesOpError("df must be positive"):
          k = matern.GeneralizedMatern(
              df=-1.0,
              amplitude=1.0,
              length_scale=1.0,
              validate_args=True,
          )
          self.evaluate(k.apply([[1.0]], [[1.0]]))
  
imports: from absl.testing import parameterized
  import numpy
  import scipy.special
  import tensorflow.compat.v2
  from tensorflow_probability.python.internal import test_util
  from tensorflow_probability.python.math import gradient
  from tensorflow_probability.python.math.psd_kernels import matern
label_desc: Use image node_id 3 for calling the GeneralizedMaternTest obj's underlying member method code with example usage: obj.testValidateDFPositive() without return types