[PATCH] Document arithmetic overflow semantics

[Robert Dewar]

> Java requires strict 32-bit two's complement arithmetic for its ints,
> and PLUS_EXPR isn't guaranteed to do that.  We could, of course,
> generate something like
> 
>    uint((ulong)a + (ulong)b)
> 
> which would be well defined. 

it seems clear that Java should be using unsigned arithmetic for all operations,
since that is the way Java is defined.