anchor.diff: -{ lib, stdenv, fetchurl, perl, perlPackages, makeWrapper, nettools, java, polyml, z3, rlwrap, makeDesktopItem }:
  +{ lib, stdenv, fetchurl, perl, perlPackages, makeWrapper, nettools, java, polyml, z3, veriT, rlwrap, makeDesktopItem }:
   # nettools needed for hostname
   
   stdenv.mkDerivation rec {
     pname = "isabelle";
  -  version = "2021";
  +  version = "2021-1";
   
     dirname = "Isabelle${version}";
   
     src = if stdenv.isDarwin
       then fetchurl {
         url = "https://isabelle.in.tum.de/website-${dirname}/dist/${dirname}_macos.tar.gz";
  -      sha256 = "1c2qm2ksmpyxyccyyn4lyj2wqj5m74nz2i0c5abrd1hj45zcnh1m";
  +      sha256 = "0n1ls9vwf0ps1x8zpb7c1xz1wkasgvc34h5bz280hy2z6iqwmwbc";
       }
       else fetchurl {
         url = "https://isabelle.in.tum.de/website-${dirname}/dist/${dirname}_linux.tar.gz";
  -      sha256 = "1isgc9w4q95638dcag9gxz1kmf97pkin3jz1dm2lhd64b2k12y2x";
  +      sha256 = "0jfaqckhg388jh9b4msrpkv6wrd6xzlw18m0bngbby8k8ywalp9i";
       };
   
     nativeBuildInputs = [ makeWrapper ];
  -  buildInputs = [ perl polyml z3 ]
  +  buildInputs = [ perl polyml z3 veriT ]
                ++ lib.optionals (!stdenv.isDarwin) [ nettools java ];
   
     sourceRoot = dirname;
  
positive.code:       Z3_VERSION=${z3.version}
        Z3_SOLVER=${z3}/bin/z3
        Z3_INSTALLED=yes
      EOF
  
      cat >contrib/polyml-*/etc/settings <<EOF
  
negative.code:     # desktop item
      mkdir -p "$out/share"
      cp -r "${desktopItem}/share/applications" "$out/share/applications"
    '';
  
    desktopItem = makeDesktopItem {
  