Add an interesting type-family/GADT example of deletion for red-black trees

Due to Stephanie Weirich, Dan Licata, John Hughes, Matt Might
Comments only
Temporary fix to the crash

   ghc-stage2: panic! (the 'impossible' happened)
     (GHC version 7.9.20140828 for x86_64-unknown-linux):
   	nameModule $w$smiddle_sfx6

   make[1]: *** [utils/haddock/dist/build/Haddock/Backends/Xhtml.dyn_o] Error 1