motion for July: clarify conversion to unsigned integer format - plan A - After rounding

[David Hough 754R work <754r@xxxxxxxxxxx>]

This was actually tabled in June, but I'll resubmit it anyway for July since
it's not on the agenda yet:

Motion: clarify conversion to unsigned integer format - plan A - After rounding

Rationale: On June 21 we discussed various approaches to this problem.
The one certain thing is that the draft is unclear.
This motion removes one aspect of that uncertainty.
The text parallels that of case 8.

Text:
In section 7.1, add an item to the list of invalid operations:

   11. Conversion of an internal floating-point number, which would 
       convert to a negative signed integer under the prevailing
       rounding mode,
       to an unsigned integer format and this cannot otherwise be indicated.

Commentary:
An alternative formulation

   11. Conversion of an internal floating-point number, strictly less than zero,
       to an unsigned integer format and this cannot otherwise be indicated.

is based on the proposition that you shouldn't be converting a negative
number to an unsigned format.
However this is unlike the case of conversion to a signed integer, which
is invariably treated as "after rounding."

An even more alternative formulation, when going to twos complement unsigned,
is to have no exceptions for finite arguments, and deliver (at potentially
extravagant cost) the correct result, modulo 2^n.