> However, as we stopped generating primes at 100, the &strong subroutine would have received an undefined value when it requested that n+1th prime. When used as a number, that undefined value would convert to zero, so the average of 97's two prime neighbours would be computed as (89+0)/2

I think I prefer Python's behaviour here, of just raising an OutOfRangeError, and not permitting NoneType to be coerced as an number.