> Thomas Fehige <mailto:thomas@fehige.de>
> 6. Juni 2016 um 10:11
> I'm trying to put a piece of text in a partial frame that's meant to 
> look like square brackets aligned with the left and right edges of the 
> textblock.
>
> I tried to use two \framed constructions inside each other, the outer 
> one with width=textwidth and negative toffset and boffset, the inner 
> one with a smaller width, a fat rulethickness and the color of the 
> background, expecting it to erase the outer frame where it's not needed:
>
> \color[zier]{%
>
> \framed[%
>
> align=middle,
>
> width=\textwidth,
>
>                         toffset=-4mm,
>
> boffset=-4mm,
>
> strut=no,
>
> ]{%
>
> \color[paper]{\framed[%
>
> align=middle,
>
> width=222mm, % i.e. \textwidth-4mm
>
>                                 toffset=6mm,
>
> boffset=5mm,
>
> rulethickness=5mm
>
> ]{%
>
> \color[zier]{\tfd \strut Here's the text that is menat to look 
> important; its length/number of lines is undetermined.\strut}
>
> }}%
>
> }%
>
> }%
>
> ... but it doesn't work. Apparently the inner frame is drawn before 
> the outer one. Putting the wider frame inside the narrower results in 
> alignment problems --  due to the overfull hbox, I presume.
Your example is of not much use because it can’t be converted to a PDF 
(missing \starttext and empty lines).
> Is  there a simple solution in ConTeXt that I haven't found yet, or 
> will I have to try some plain TeX box magic myself?
You can use MetaPost to draw the frame/lines for the left and right borders.

\startuseMPgraphic{leftbracket}
     z[0] = (EmWidth,OverlayHeight) ;
     z[1] = (0,OverlayHeight) ;
     z[2] = (0,0) ;
     z[3] = (EmWidth,0) ;
     draw z[0]--z[1]--z[2]--z[3] withcolor \MPcolor{\overlaylinecolor} ;
\stopuseMPgraphic

\startuseMPgraphic{rightbracket}
     z[0] = (-EmWidth,OverlayHeight) ;
     z[1] = (0,OverlayHeight) ;
     z[2] = (0,0) ;
     z[3] = (-EmWidth,0) ;
     draw z[0]--z[1]--z[2]--z[3] withcolor \MPcolor{\overlaylinecolor} ;
\stopuseMPgraphic

\installleftframerenderer {bracket}{\useMPgraphic {leftbracket}}
\installrightframerenderer{bracket}{\useMPgraphic{rightbracket}}

\defineframed
   [bracketframed]
   [frame=off,
    offset=1ex,
    leftframe=bracket,
    rightframe=bracket,
    framecolor=red,
    foregroundstyle=\ssx,
    foregroundcolor=darkgray,
    align=normal,
    width=max]

\starttext
\bracketframed{\input knuth }
\stoptext

Wolfgang