I am using the python api and when my validation callback is triggered i want to access the opaque string i have passed with the tailf:opaque statement in the yang model. The problem is that i cannot find a way to do that when using the transaction context. I keep getting AttributeError when trying to access the '_confd.TransCtxRef '. What is the name of the attribute i must use? I have tried v_opaque, validate_opaque and callpoint_opaque. The user guide does not refer how to do that when using the python api.