[luatex] New over/under delimiter primitives

[Philipp Stephani]

Hello,

I have a general question about the primitives \Uoverdelimiter, 
\Uunderdelimiter, \Udelimiterover and \Udelimiterunder. ConTeXt seems to use the 
two former in math-ini.lua:

local function mathtopdelimiter(class,family,slot)
    return format('\\Uoverdelimiter "%X "%X ',0,family,slot) -- no class
end
local function mathbotdelimiter(class,family,slot)
    return format('\\Uunderdelimiter "%X "%X ',0,family,slot) -- no class
end

where the slot argument is unused?!
I also tried to play around with them in plain LuaTeX:

\input luaotfload.sty
\font\cambria={name:Cambria Math:script=math} at 11pt
\textfont0=\cambria
\def\overrightarrow{\Uoverdelimiter 0 "2192 }
\def\underrightarrow{\Uunderdelimiter 0 "2192 }
\def\xoverrightarrow{\Udelimiterover 0 "2192 }
\def\xunderrightarrow{\Udelimiterunder 0 "2192 }
\def\abc{\Umathchar 0 0 "1D44E \Umathchar 0 0 "1D44F \Umathchar 0 0 "1D450 }
$$ \abc \quad \overrightarrow{\abc} \quad \underrightarrow{\abc} $$
\bye

That results in "abc → →", without any subscripts or superscripts. So what do 
these primitives actually do, and how should we use them?

Regards,
Philipp