[luatex] \Uleft / \Uright problems

[Ulrike Fischer]

The documentation describes \Uleft / \Uright (also) as an
alternative to the plain \big, \bigl ... commands. 

But \bigl can be used on its own, while \Uleft requires an matching
\Uright: 

$\bigl\{ x $  %works
$\Uleft height 10pt depth 10pt axis \{ x $ %error
 
Would it be possible to extend the commands so that they can be used
alone?


Another problem is the size of the delimiters, which is sometimes
not really as expected. As an example a plain tex test with legacy
math fonts:

\newdimen\size \size 21pt
\def\test{
  \ifdim\size<4.5pt \cdot
  \else
    \Uleft  height \size depth \size axis  |
    {\advance\size -1pt \test}
    \Uright height \size depth \size axis  |
  \fi}

$\test$

\bye

or context and cambria:

\setupbodyfont[cambria]

\starttext
\newdimen\size \size 21pt
\def\test{
  \ifdim\size<4.5pt \cdot
  \else
    \Uleft  height \size depth \size axis  |
    {\advance\size -1pt \test}
    \Uright height \size depth \size axis  |
  \fi}

$\test$
\stoptext


Why are the size so varied (using the "exact" keyword doesn't change
the output here)?

-- 
Ulrike Fischer 
https://www.troubleshooting-tex.de/