What does %% do? The documentation says: a %% b is the same as (a % b + b) % b, but what is the difference between "true mathematical modulo" and the standard modulo?
One reason this is useful is if you want an array to wrap around. With modulo, you just index on `index % length` and all is well. But with remainder, this will only do what you want for `index >= 0`, because it can give negative outputs, given negative inputs.