def forward_sync(self):
    """A control trigger node for synchronization in the forward loop.

    One main use is to keep the push ops of a stack executed in the
    iteration order.
    """
    if self._forward_sync is None:
      with ops.control_dependencies(None):
        self._forward_sync = control_flow_ops.control_trigger(name="f_sync")
      self._forward_sync._set_control_flow_context(self._forward_context)
      self._forward_index.op._add_control_input(self._forward_sync)
    return self._forward_sync
  def grad_sync(self):
    """A control trigger node for synchronization in the grad loop.

    One main use is to keep the pop ops of a stack executed in the
    iteration order.
    """
    if self._grad_sync is None:
      with ops.control_dependencies(None):
        self._grad_sync = control_flow_ops.control_trigger(name="b_sync")
      self._grad_sync._set_control_flow_context(self._grad_context)
      self._grad_index.op._add_control_input(self._grad_sync)
      if self._grad_context.outer_context:
        self._grad_context.outer_context.AddInnerOp(self._grad_sync)
    return self._grad_sync