[luatex] "Proper" use of \Udelimiterover &c

[Will Robertson]

Hi,

Just checking that I'm using these primitives correctly.

Plain & LaTeX have some complex alignment definitions for \overbrace 
and friends; am I correct in thinking that \Udelimiterover are designed 
to replace all of that?

Khaled's minimal example is:

\input luaotfload.sty
\font\4={name:XITS Math:mode=base;script=math} at 10pt
\textfont4=\4

$$
  \Udelimiterover  "4 "23DE {a+b}
+ \Udelimiterunder "4 "23DF {a+b} = C
$$

But here the spacing is much too large between the delimiter and its 
contents. Am I supposed to be boxing up/smashing/whatever the argument 
myself?

If not, and it's just a matter of setting the delimitervgap 
appropriately, I'm curious why (philosophically) the 
\mathop{...}\limits construction is required:

\Umathoverdelimitervgap\displaystyle=-3pt % ?
$$
  \mathop{\Udelimiterover  "4 "23DE {a+b}}\limits^{d+e}
+ \Udelimiterunder "4 "23DF {a+b} = C
$$

Would it make sense to treat a \Udelimiterover construction as an 
operator with limits automatically? E.g., I expected

{\Udelimiterover  "4 "23DE {a+b}}^{d+e}

to do the "right thing", but looking at it this way perhaps it's 
impossible to do this with this construction of maths primitives? 
(Sorry I'm not entirely up to speed on the internals of the maths 
engine.)

Cheers,
Will