g mode to UPWARD or DOWNWARD, it is better to set it 
immediately back to NEAREST. However  we have no guarantee 
on how the compiler will reorder the instructions generated. 
It is ALWAYS better to write:
<p>

let a = set_high(); let res = 1./.3. in set_nearest (); res;;
<p>

The above code will NOT work on linux-x64 where many floating point 
functions are implemented using SSE instructions.
These three functions should only be used when there is no other
solution, and you really know what tou are doing, 