anchor.diff:              tests.cp2k
               tests.nwchem
               tests.molcas
  +          ] ++ pkgSet.lib.optionals allowUnfree [
               molden
  -          ] ++ pkgSet.lib.optionals (cfg.srcurl != null) [
  +          ] ++ pkgSet.lib.optionals (cfg.srcurl != null && allowUnfree) [
               tests.molpro
               tests.mesa-qc
               tests.qdng
  
positive.code: 
    # Override config from ENV
    , config ? {}
    , NixOS-QChem ? { shortRev = "0000000"; }
    # build more variants
    , buildVariants ? false
  
negative.code:   # Customized package set
    pkgs = config: overlay: let
      pkgSet = (import nixpkgs) {
        overlays = [ overlay ] ++ preOverlays ++ [ (import ./default.nix) ] ++ postOverlays;
        config.allowUnfree = allowUnfree;
        config.qchem-config = cfg;
      };
  
  