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. Thanks, Aditya