appendix G, rule 12: \rule 12. If the current item is an Acc atom (from ^|\mathaccent|), just
go to Rule~16 if the accent character doesn't exist in the current size.
Otherwise set box~$x$ to the nucleus in style $C'$, and set $u$ to the
width of this box. If the nucleus is not a single character, let $s=0$;
otherwise set $s$ to the kern amount for the nucleus followed by the
^|\skewchar| of its font. If the accent character has a successor in its
font whose width is $\le u$, change it to the successor and repeat this