I think the challenge is it isn't completely obvious to me that starting with 0 is pedagogically correct, though it is so logically. `y >= x + y` is different from `y != x + y`; the former says that addition (of natural numbers) is monotonically increasing, the latter that there is no identity element. Is it possible that admitting an identity element is confusing for beginners? Or is this just a consequence of my own educational conditioning?

(For the record I think all the arguments Dijkstra gives for 0-indexing are correct. I just lack the mathematical intuition to see why the special case above appears to be one in which admission of 0 as complicates things.)