anchor.diff:    args = rec {
       pkgs = import <nixpkgs> {};
       pythonPackages = pkgs.python36Packages;
  -    forTest = true;
  +    forDev = true;
       localOverridesPath = ./local.nix;
     } // argsOuter;
   in (with args; {
  
positive.code:         ${pythonPackages.virtualenv}/bin/virtualenv $VIRTUALENV_ROOT
        fi
        source $VIRTUALENV_ROOT/bin/activate
        make requirements${pkgs.stdenv.lib.optionalString forTest "_for_test"}
      '';
    }).overrideAttrs (if builtins.pathExists localOverridesPath then (import localOverridesPath args) else (x: x));
  })
  
negative.code:       pkgs.git
        # for `cryptography`
        pkgs.openssl
      ] ++ pkgs.stdenv.lib.optionals forDev ([
          # for lxml
          pkgs.libxml2
          pkgs.libxslt
  