It's mentioned later, but the "math friendly" versions satisfy the axioms of a group and a field, which only works if the modulus is a prime.

In particular, all non-zero values have a unique inverse:

> As 251 is prime, this reduction rule is math-friendly. By that, I mean that for any x other than zero, there is some y such that x * y == 1. For example, taking x of 16, we have 16 * 204 == 1. This property is not true for the 256 == 0 reduction rule; with that rule, there is no y such that 16 * y == 1. Where it exists, this y can be called inv(x) or 1/x.