code: def test_backend_can_be_set(self):
      self.assertEqual(fastmath.backend_name(), "jax")
      fastmath.set_backend("tensorflow-numpy")
      self.assertEqual(fastmath.backend_name(), "tensorflow-numpy")
      fastmath.set_backend(None)
      self.assertEqual(fastmath.backend_name(), "jax")
  
imports: import collections
  from absl.testing import parameterized
  import gin
  import jax.numpy
  import numpy
  from tensorflow import test
  from trax import fastmath
label_desc: Use image node_id 4 for calling the BackendTest obj's underlying member method code with example usage: obj.test_backend_can_be_set() without return types