anchor.diff:  
   ## disable prompts from apt
   ENV DEBIAN_FRONTEND noninteractive
  +
  +## custom apt-get install options
  +ENV OPTS_APT        -y --force-yes --no-install-recommends
   
   ## ensure locale is set during build
   ENV LC_ALL          en_US.UTF-8
  
positive.code: 
  ## install llvm for the ghc backend
  RUN apt-get update\
   && apt-get install -y --force-yes --no-install-recommends llvm
  
  ## haskell package versions; can be overriden via context hacks
  ENV VERSION_ALEX   3.1.3
  
negative.code: 
  ## install minimal set of haskell packages
  RUN apt-get update\
   && apt-get install ${OPTS_APT}\
        alex-"${VERSION_ALEX}"\
        cabal-install-"${VERSION_CABAL}"\
        ghc-"${VERSION_GHC}"\
  