old_contents: from ._backend import Backend, assert_backend_available
  from .. import make_tracing_backend_decorator
  
  
  class CallableBackend(Backend):
      def __str__(self):
          return "callable"
  
      @staticmethod
      def is_available():
          return True
  
      @assert_backend_available
      def is_compatible(self, objective, argument):
          return callable(objective)
  
      @assert_backend_available
      def compile_function(self, objective, argument):
          return objective
  
      @assert_backend_available
      def __not_implemented(self, objective, argument):
          raise NotImplementedError("No autodiff support available for the "
                                    "canonical callable backend")
  
      compute_gradient = compute_hessian = __not_implemented
  
  
  Callable = make_tracing_backend_decorator(CallableBackend)
  
new_contents: from ._backend import Backend, assert_backend_available
  from .. import make_tracing_backend_decorator
  
  
  class _CallableBackend(Backend):
      def __str__(self):
          return "callable"
  
      @staticmethod
      def is_available():
          return True
  
      @assert_backend_available
      def is_compatible(self, objective, argument):
          return callable(objective)
  
      @assert_backend_available
      def compile_function(self, objective, argument):
          return objective
  
      def _raise_not_implemented_error(self, objective, argument):
          raise NotImplementedError("No autodiff support available for the "
                                    "canonical 'Callable' backend")
  
      compute_gradient = compute_hessian = _raise_not_implemented_error
  
  
  Callable = make_tracing_backend_decorator(_CallableBackend)
  
subject: clean up variable/class names