% transformations.
%
+\startuniqueMPgraphic{box}
+path a, b;
+% Create a rectangle of the full size
+a := unitsquare xyscaled(\overlaywidth,\overlayheight);
+% Randomize it a bit
+b := a randomized 10pt ;
+% And draw it
+drawoptions (withpen pencircle scaled .75mm withcolor black) ;
+draw b;
+\stopuniqueMPgraphic
+
+\defineoverlay[box][\uniqueMPgraphic{box}]
+
+% Define \{start,stop}boxed with a nice metapost box around it.
+\defineframedtext[boxed][background=box,frame=off]
+% A boxed variant with the width forced to page width. This is used in
+% \startrans, since the content somehow doesn't scale the box around it
+% properly.
+\defineframedtext[wideboxed][background=box,frame=off,width=\textwidth]
+
% Install the lambda calculus pretty-printer, as defined in pret-lam.lua.
\installprettytype [LAM] [LAM]
% Define \startlambda \stoplambda
-\definetyping[lambda][option=LAM,style=sans]
+\definetyping[lambda][option=LAM,style=sans,before=\startboxed,after=\stopboxed]
+
% Define \lam{} (since \lambda is taken)
\definetype[lam][option=LAM,style=sans]
% Install the transformation pretty-printer, as defined in pret-trans.lua.
\installprettytype [TRANS] [TRANS]
% Define \starttrans \stoptrans
-\definetyping[trans][option=TRANS,style=normal,before=]
+\definetyping[trans][option=TRANS,style=normal,before=\startwideboxed,after=\stopwideboxed]
% Install the haskell pretty-printer, as defined in pret-haskell.lua.
\installprettytype [HASKELL] [HASKELL]
% Define \starthaskell \stophaskell
-\definetyping[haskell][option=HASKELL,before=,after=]
+\definetyping[haskell][option=HASKELL,before=\startboxed,after=\stopboxed]
% Define \hs
\definetype[hs][option=HASKELL,style=mono]