Don't underflow

Reason

Decrementing a value beyond a minimum value can lead to memory corruption and undefined behavior.

Example, bad

int a[10];
a[-2] = 7;   // bad

int n = 101;
while (n--)
    a[n - 1] = 9;   // bad (twice)

Exception

Use unsigned types if you really want modular arithmetic.

Enforcement

???