\environment xits-env

\startbuffer[THEFORMULA]
   \left\Vert\left\vert \frac12 \right\vert \right\Vert
   \vert\big\vert\Big\vert\bigg\vert\Bigg\vert
\stopbuffer

\starttext

\startTEXpage
  \startformula
    \getbuffer[THEFORMULA]
  \stopformula
\stopTEXpage

\stoptext
