So, my problems with the rfc:
* if does not know nor want to know about input-only or output-only pins
* as a consequence, an output-only pin will work in INPUT_ONLY mode...  until a later hardware revision makes it bidir somehow
* and an output only pin will show mode INPUT_ONLY by default, and will ignore its mode setup
* while an input only pin will (by chance) show INPUT_ONLY, but will also ingore the mode setup, while pretending (when reading back the register) that you actually changed something