"Ah. I suppose I owe the tropes an apology, then."
"I will look about those priests who might otherwise be suitable to take my place, to see if there is a mathematical adept among them. If you can in fact teach them to be my suitable successor, you may have your reward the day after, as you will, or at the end of your days after a suitably luxurious life."
The mission is not actually optional, nor would failure at it be tolerated, but these things need not be said aloud while Sevar's strange experiment is in progress.