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