anchor.diff:    # Customized package set
     pkgs = config: overlay: let
       pkgSet = (import nixpkgs) {
  -      overlays = [ overlay (import ./default.nix) ];
  -      config.allowUnfree = true;
  +      overlays = [ overlay ] ++ preOverlays ++ [ (import ./default.nix) ] ++ postOverlays;
  +      config.allowUnfree = allowUnfree;
         config.qchem-config = cfg;
       };
   
  
positive.code:             tests.cp2k
              tests.nwchem
              tests.molcas
              molden
            ] ++ pkgSet.lib.optionals (cfg.srcurl != null) [
              tests.molpro
              tests.mesa-qc
              tests.qdng
  
negative.code: 
    # 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
  