[texhax] Peculiar behavior of \mathaccent

[Barbara Beeton]

    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?

the cmmi "7F is what is used for \t -- a
2-character "tie".  see texbook, p.52.
							-- bb