Hello Hans,
The macro to draw a vector does not work even with the latest version of context standalone.
Thank you
Fabrice

\useMPlibrary[mat]

% will be in meta-imp-mat.mkiv

\startMPextensions
     vardef math_stacker_arrow_shape =
         image (
             drawarrow
                 (OverlayWidth,OverlayOffset) -- (0,OverlayOffset)
             withcolor
                 OverlayLineColor ;
             setbounds currentpicture to boundingbox currentpicture
bottomenlarged (OverlayOffset/2) topenlarged (OverlayOffset/2) ;
         )
     enddef ;
     vardef math_stacker_leftarrow_shape =
         math_stacker_arrow_shape
     enddef ;
     vardef math_stacker_rightarrow_shape =
         math_stacker_arrow_shape rotated 180
     enddef ;
\stopMPextensions

\startuniqueMPgraphic{math:stacker:\number"2190}
     math_stacker_draw(math_stacker_leftarrow_shape) ;
\stopuniqueMPgraphic

\startuniqueMPgraphic{math:stacker:\number"2192}
     math_stacker_draw(math_stacker_rightarrow_shape) ;
\stopuniqueMPgraphic

% so far

\setupmathstackers
   [both] % vfenced]
   [color=darkred,
    alternative=mp]

\setupmathstackers
   [top]
   [color=darkred,
    alternative=mp]

\setupmathstackers
   [bottom]
   [color=darkred,
    alternative=mp]

\starttext

$\overbracket   {a+b+c+d} \quad \underbracket    {a+b+c+d} \quad
\doublebracket{a+b+c+d}$ \blank
$\overparent    {a+b+c+d} \quad \underparent     {a+b+c+d} \quad
\doubleparent {a+b+c+d}$ \blank
$\overbrace     {a+b+c+d} \quad \underbrace      {a+b+c+d} \quad
\doublebrace  {a+b+c+d}$ \blank
$\overbar       {a+b+c+d} \quad \underbar        {a+b+c+d} \quad
\doublebar    {a+b+c+d}$ \blank

$\overleftarrow  {a+b+c+d} \quad \overrightarrow  {a+b+c+d} $ \blank
$\underleftarrow {a+b+c+d} \quad \underrightarrow {a+b+c+d} $ \blank

\stoptext