Initially I thought that the 'prettyprinter' package could serve that 
purpose. But their 'emptyDoc' is not a unit for vertical composition, which 
is something we pretty much rely on (IIRC). We also do not need rendering 
based on document width etc. However, I do like their SimpleDocStream type, 
not least because the 'terminal' package provides a backend to render it to 
a terminal with colors etc. So perhaps we could re-write our 
Darcs.Util.Printer to create a SimpleDocStream.