code: def test_smoothing_rate_is_immutable(self):
      with pytest.raises(AttributeError):
          self.instance.b = None
      with pytest.raises(AttributeError):
          self.instance.smoothing_rate = None
  
imports: import pytest
  from sympy import Symbol
  from sympy.core.numbers import Float, Integer, Rational
  from sympy.functions.elementary.hyperbolic import tanh
  from sympy.matrices import Matrix
  from sympy.matrices.dense import zeros
  from sympy.physics.mechanics import dynamicsymbols
  from sympy.physics.biomechanics import ActivationBase, FirstOrderActivationDeGroote2016, ZerothOrderActivation
  from sympy.physics.biomechanics._mixin import _NamedMixin
  from sympy.simplify.simplify import simplify
label_desc: Use image node_id 16 for calling the TestFirstOrderActivationDeGroote2016 obj's underlying member method code with example usage: obj.test_smoothing_rate_is_immutable() without return types