Accents in the operator dictionary

So questions/remarks:

1) What is the rationale for defining most accents as "postfix" operators?
2) The rules for determining the form and the operator dictionary make 
this two cases behave differently:
   - low line: <mover><mi>abcdef</mi><mo>_</mo></mover> (form = infix, 
so not stretchy)
   - overline: <mover><mi>abcdef</mi><mo>&#x203E;</mo></mover> (form = 
postfix, so stretchy)
3) Why is "check mark" a stretchy accent (or even just a mathematical 
operator)? Is it related to \widecheck (caron) or square root?

-- 
Frédéric Wang.
www.maths-informatique-jeux.com <http://www.maths-informatique-jeux.com>
www.maths-informatique-jeux.com/international 
<http://www.maths-informatique-jeux.com/international>