Wiggy, yes, I know all that. What I was commenting on was the poster's apparent belief that an airline would nip back to base from well into a sector solely, repeat solely, to collect a pilot to replace a crew member on the next flight from the destination. Of course it's mandatory to divert/RTB if a member of the flight deck operating crew is incapacitated or likely to become so during a flight. At least I have always believed it is.