`let _left=40-$_done` throws an error, when `set -e` and `$_done = 40`

[OlJohnny]

When one adds set -e in their environment/script, the code line let _left=40-$_done errors out, when 40-$_done equals 0.

In my testing let _left=(40-$_done) || printf 0 works perfectly fine