Say you have a register whose range is -9..+9. (5-1)+5 doesn't trap but (5+5)-1 does.

I understand how to make an example of this; the question is why should (5-1)+5 trap, just because (5+5)-1 does.

Ah, good point! I was thinking of unsigned overflow, but signed overflow is a different matter, as pointed out elsewhere in this thread. I retract my statement about trapping overflow.