Hi Hans,
The console returns me :

resolvers       | trees | analyzing 'home:texmf'
mtx-context     | run 1: luatex --fmt="/home/fab/context/tex/texmf-cache/luatex-cache/context/5fe67e0bfe781ce0dde776fb1556f32e/formats/luatex/cont-en" --jobname="ntg45" --lua="/home/fab/context/tex/texmf-cache/luatex-cache/context/5fe67e0bfe781ce0dde776fb1556f32e/formats/luatex/cont-en.lui" --no-parse-first-line --c:currentrun=1 --c:fulljobname="./ntg45.tex" --c:input="./ntg45.tex" --c:kindofrun=1 --c:maxnofruns=9 "cont-yes.mkiv"
This is LuaTeX, Version 0.95.0 (TeX Live 2016)
 system commands enabled.

resolvers       > trees > analyzing 'home:texmf'
open source     > 1 > 1 > /home/fab/context/tex/texmf-context/tex/context/base/mkiv/cont-yes.mkiv

ConTeXt  ver: 2016.06.05 16:26 MKIV beta  fmt: 2016.6.6  int: english/english

system          > 'cont-new.mkiv' loaded
open source     > 2 > 2 > /home/fab/context/tex/texmf-context/tex/context/base/mkiv/cont-new.mkiv
close source    > 2 > 2 > /home/fab/context/tex/texmf-context/tex/context/base/mkiv/cont-new.mkiv
system          > files > jobname 'ntg45', input './ntg45', result 'ntg45'
fonts           > latin modern fonts are not preloaded
languages       > language 'en' is active
open source     > 2 > 3 > /home/fab/ntg45.tex
metapost        > library 'mat' is unknown or invalid
fonts           > preloading latin modern fonts (second stage)
fonts           > 'fallback modern-designsize rm 12pt' is loaded
metapost        > initializing instance 'metafun' using format 'metafun'
metapost        > loading 'metafun' as '/home/fab/context/tex/texmf-context/metapost/context/base/mpiv/metafun.mpiv' using method 'default'
metapost        > initializing number mode 'scaled'
metapost        > error:





>> math_stacker_draw
! Isolated expression.
<to be read again>
                   (
<*>  math_stacker_draw(
                       math_stacker_leftarrow_shape) ; ;
! Extra tokens will be flushed.
<to be read again>
                   (
<*>  math_stacker_draw(
                       math_stacker_leftarrow_shape) ; ;

[1]

no-error
metapost        > error:



>> math_stacker_draw
! Isolated expression.
<to be read again>
                   (
<*>  math_stacker_draw(
                       math_stacker_leftarrow_shape) ; ;
! Extra tokens will be flushed.
<to be read again>
                   (
<*>  math_stacker_draw(
                       math_stacker_leftarrow_shape) ; ;

[1]

no-error
metapost        > error:




>> math_stacker_draw
! Isolated expression.
<to be read again>
                   (
<*>  math_stacker_draw(
                       math_stacker_rightarrow_shape) ; ;
! Extra tokens will be flushed.
<to be read again>
                   (
<*>  math_stacker_draw(
                       math_stacker_rightarrow_shape) ; ;

[1]

no-error
metapost        > error:



>> math_stacker_draw
! Isolated expression.
<to be read again>
                   (
<*>  math_stacker_draw(
                       math_stacker_rightarrow_shape) ; ;
! Extra tokens will be flushed.
<to be read again>
                   (
<*>  math_stacker_draw(
                       math_stacker_rightarrow_shape) ; ;

[1]

no-error
backend         > xmp > using file '/home/fab/context/tex/texmf-context/tex/context/base/mkiv/lpdf-pdx.xml'
pages           > flushing realpage 1, userpage 1
close source    > 2 > 3 > /home/fab/ntg45.tex
close source    > 1 > 3 > /home/fab/context/tex/texmf-context/tex/context/base/mkiv/cont-yes.mkiv

mkiv lua stats  > used config file: selfautoparent:/texmf/web2c/texmfcnf.lua
mkiv lua stats  > used cache path: /home/fab/context/tex/texmf-cache/luatex-cache/context/5fe67e0bfe781ce0dde776fb1556f32e
mkiv lua stats  > resource resolver: loadtime 0.032 seconds, 1 scans with scantime 0.021 seconds, 0 shared scans, 22 found files, scanned paths: /home/fab/texmf
mkiv lua stats  > stored bytecode data: 390 modules (0.281 sec), 82 tables (0.011 sec), 472 chunks (0.292 sec)
mkiv lua stats  > traced context: maxstack: 1182, freed: 2, unreachable: 1180
mkiv lua stats  > cleaned up reserved nodes: 48 nodes, 9 lists of 446
mkiv lua stats  > node memory usage: 139 glue, 2 penalty, 180 attribute, 38 glue_spec, 34 attribute_list, 2 temp, 2 user_defined
mkiv lua stats  > node list callback tasks: 6 unique task lists, 6 instances (re)created, 88 calls
mkiv lua stats  > used backend: pdf (backend for directly generating pdf output)
mkiv lua stats  > jobdata time: 0.001 seconds saving, 0.000 seconds loading
mkiv lua stats  > callbacks: 246 direct, 303 indirect, 549 total
mkiv lua stats  > randomizer: resumed with value 0.28739196541132
mkiv lua stats  > positions: 8 collected, 8 regulars, 0 regions, 0 unresolved regions
mkiv lua stats  > loaded patterns: en::2, load time: 0.000
mkiv lua stats  > result saved in file: ntg45.pdf, compresslevel 3, objectcompresslevel 3
mkiv lua stats  > loaded fonts: 2 files: latinmodern-math.otf, lmroman12-regular.otf
mkiv lua stats  > font engine: otf 3.022, afm 1.512, tfm 1.000, 4 instances, load time 0.149 seconds
mkiv lua stats  > used platform: linux-64, type: unix, binary subtree: texmf-linux-64
mkiv lua stats  > luatex banner: this is luatex, version 0.95.0 (tex live 2016)
mkiv lua stats  > control sequences: 43841 of 65536 + 100000
mkiv lua stats  > lua properties: engine: lua, used memory: 59 MB (ctx: 58 MB), hash type: lua, hash chars: min(64,40), symbol mask: utf (τεχ)
mkiv lua stats  > runtime: 0.551 seconds, 1 processed pages, 1 shipped pages, 1.815 pages/second

system          | total runtime: 0.784 seconds


Thanks,
Fabrice

2016-06-06 22:50 GMT+02:00 Hans Hagen <pragma@wxs.nl>:
On 6/6/2016 9:39 PM, Fabrice Couvreur wrote:
Hi Joseph,
This is already the command I used, but I think the point of the too
small arrow.

If you really want to go over the top:

\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


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                                          Hans Hagen | PRAGMA ADE
              Ridderstraat 27 | 8061 GH Hasselt | The Netherlands
      tel: 038 477 53 69 | www.pragma-ade.com | www.pragma-pod.nl
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