Hi,

The following example fails:

\usemodule[pgfplots]
\usepgfplotslibrary[fillbetween]

\starttext
\startformula
\left. a \right|
\stopformula

\stoptext

with

tex error       > tex error on line 7 in file /private/tmp/test.tex: ! Missing \endgroup inserted

<inserted text>
\endgroup
<to be read again>
\Ucheckedstopdisplaymath
\stopdisplaymath ...math \Ucheckedstopdisplaymath
                                                  \par \ifvmode \ifcase \c_s...
\strc_formulas_stop_formula ...native \v!formula }
                                                  \dostoptagged \dostoptagge...
l.7 \stopformula

This is a weird bug. Only \left. \right| fails; \left| \right| works correctly. If I remove the fillbetween library, the error goes away.

Any hints as to what is happening will be appreciated.