[MathML3-last-call] clarify what happens if mscarries is last row of mstack/mlongdiv

The spec is silent on what should happen if the last row of an mstack is an
mscarries that has 'location' not equal to 'n' (ie, that wants to display as
an adornment to to the following row).  A similar problem happens when
'crossout' is not equal to 'none' (nothing to cross out).

I propose adding the following paragraph to the end of 3.6.5.2:

If mscarries is the last row in an mstack or mlongdiv, then any carries or
crossouts that would normally be drawn on the following line are ignored and
not should not be drawn.

Neil Soiffer
Senior Scientist
Design Science, Inc.
www.dessci.com
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