anchor.diff:              --subst-var-by nix ${toplevel} \
               --subst-var-by cacert ${cacert}
   
  -          shellcheck -e SC1090 $TMPDIR/install
  -          shellcheck -e SC1091,SC2002 $TMPDIR/install-darwin-multi-user
  +          if type -p shellcheck; then
  +            shellcheck -e SC1090 $TMPDIR/install
  +            shellcheck -e SC1091,SC2002 $TMPDIR/install-darwin-multi-user
  +          fi
   
             chmod +x $TMPDIR/install
             chmod +x $TMPDIR/install-darwin-multi-user
  
positive.code: { nix ? builtins.fetchGit ./.
  , nixpkgs ? fetchTarball channel:nixos-17.09
  , officialRelease ? false
  , systems ? [ "x86_64-linux" "i686-linux" "x86_64-darwin" "aarch64-linux" ]
  }:
  
negative.code: 
      binaryTarball = pkgs.lib.genAttrs systems (system:
  
        with import nixpkgs { inherit system; };
  
        let
  