[texhax] Peculiar behavior of \mathaccent

[Michael Barr]

I am involved in a project to reprint a 40 year old volume of Lecture 
Notes in Math.  One of the authors used an arc over a letter for 
something.  Although the character appears at position "7F in the mit 
font, I could not find a macro that implemented it.  I tried defining 
\def\arc{\mathaccent"717F }
It sort of worked.  Note that it leads to some incomprehensible error 
message without that space at the end.  However, the arc was almost 
hanging off the right edge of the character (I tested it with $\arc A$).  
It wasn't perfect, but I figured that it would do.  Then I found a double 
arc over some character.  Trying $\arc\arc A$ gave an error and $\arc{\arc 
A}$ put the second arc centered over the whole, that is offset to the left 
from the first and really looked awful.  Finally, I 
tried
\def\Arc{\mathaccent"717F }
\def\arc#1{\Arc{\hbox{$#1$}}}
This works.  $\arc A$ now centers the arc over the A and $\arc{\arc A}$ 
now puts the two arcs one above the other.  So my problem is solved, but 
can anyone explain what is going on here, why the simple macro didn't 
work?

Michael Barr