anchor.diff:    postInstall = ''
       for n in "$out/bin/"*; do wrapProgram "$n" --suffix MAUDE_LIB ':' "$out/share/maude"; done
       unzip ${fullMaude}
  -    install -D -m 444 full-maude3.maude $out/share/maude/full-maude.maude
  +    install -D -m 444 full-maude31.maude $out/share/maude/full-maude.maude
     '';
   
     # bison -dv surface.yy -o surface.c
  
positive.code: 
  let
  
    version = "3.0";
  
    fullMaude = fetchurl {
      url = "http://maude.cs.illinois.edu/w/images/0/04/Full-Maude-${version}.zip";
      sha256 = "0gf36wlkkl343vlxgryqdhxmgyn8z0cc2zayccd7ac3inmj1iayw";
    };
  
  in
  
negative.code:   inherit version;
  
    src = fetchurl {
      url = "http://maude.cs.illinois.edu/w/images/d/d3/Maude-${version}.tar.gz";
      sha256 = "b112d7843f65217e3b5a9d40461698ef8dab7cbbe830af21216dfb924dc88a2f";
    };
  
    buildInputs = [
  