[PATCH] Document arithmetic overflow semantics

[Robert Dewar]

> But undefined overflow is a superset of two's complement semantics

I don't understand the terminology here. Are you using "two's comlement" to
imply unsigned wrap around, i.e. throw away carry and ignore overflow. If so,
that's a bit confusing, since it presupposes the conclusion. The issue with
twos complement signed arithmetic is preciesely what happens on overflow (that's
the only thing that distinguishes it from unsigned).