at the moment nothing, because I don't want to make it any more broken.
an example is in order to test whether it was working as an AIN, 10k potentiometer across 5V/GND and brush to the pin. Any of the other analog input pins I had a value proportional to how far it was turned but A0 got 5V unless it was right at the one end. basically A0 is shorted to 5V...
found this when googling 10 Ways to Destroy an Arduino — Rugged CircuitsRugged Industrial Arduino Microcontrollers, I assumed the 5.6V zener would be fine but if the internal protection diode turns on at 5.5V that would explain why it is now broken and probably why it is shorted to 5V (burned right through diode)
maybe I should go fix that...