\useMPlibrary[mat]

\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
