Hello everybody, This is more like a "metaphysical " question...

To give a metaphysical answer...

Since the first triggered value of nextIndexMod: is 0, its value just *before* it is triggered should be one smaller than zero or (0 - 1). Since you've asked for the result mod 16, the indexes count up from zero and, when the index gets to 15, the next value is not 16 but 0. So the number just prior to 0 is actually 15.