Thank you Wolfgang, that is exactly what I was looking for. I apologize for the incomplete example. I have no idea, however, how all the empty lines got in there. They weren't there when I clicked "send", only when I saw it in the list. Thomas Am 06.06.2016 um 12:32 schrieb Wolfgang Schuster:
\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