protected fun onMutation(): Unit
This is used internally by generated models to do validation checking when "validateEpoxyModelUsage" is enabled and the model is used with an EpoxyController. This method validates that it is ok to change this model. It is only valid if the model hasn't yet been added, or the change is being done from an EpoxyController.Interceptor callback.
This is also used to stage the model for implicitly adding it, if it is an AutoModel and implicit adding is enabled.