The `((x++))` syntax is shorthand for `let x++`. According to `help let`:
If the last ARG evaluates to 0, let returns 1; 0 is returned
otherwise.
Thus the exit status of the expression `x=0; let x++` is 1, since the post-increment `++` operator evaluates to the value of the variable before incrementing.
In Bash 4, this non-zero exit status properly triggers `set -e`'s error trap, but in Bash 3 it does not. That's why the tests were passing on OS X (Bash 3) but not Linux (Bash 4).
We can work around the problem by choosing an incrementation expression that never evaluates to 0, such as `+=` or the pre-increment `++` operator. For consistency and clarity, I've changed to `x+=1` everywhere.
Ref. #25, #27
