+/**
+ * Register a decoder output callback function.
+ *
+ * The function will be called when a protocol decoder sends output back
+ * to the PD controller (except for Python objects, which only go up the
+ * stack).
+ *
+ * @param output_type The output type this callback will receive. Only one
+ * callback per output type can be registered.
+ * @param cb The function to call.
+ * @param cb_data Unused.
+ */