anchor.diff:        pos' = if pos != null then "‘" + pos.file + ":" + toString pos.line + "’" else "«unknown-file»";
   
         throwEvalHelp = unfreeOrBroken: whatIsWrong:
  -        assert (unfreeOrBroken == "Unfree" || unfreeOrBroken == "Broken");
  +        assert (builtins.elem unfreeOrBroken [ "Unfree"
  +                                               "Broken"
  +                                               "BlacklistedLicense"
  +                                             ]);
           throw ''
             Package ‘${attrs.name}’ in ${pos'} ${whatIsWrong}, refusing to evaluate.
             For `nixos-rebuild` you can set
  
positive.code: let
  
    allowUnfree = config.allowUnfree or false || builtins.getEnv "NIXPKGS_ALLOW_UNFREE" == "1";
  
    # Alow granular checks to allow only some unfree packages
    # Example:
  
negative.code:             { allow${unfreeOrBroken} = true; }
            to ~/.nixpkgs/config.nix.
          '';
  
        # Check whether unfree packages are allowed and if not, whether the
        # package has an unfree license and is not explicitely allowed by the
        # `allowUNfreePredicate` function.
        hasDeniedUnfreeLicense = attrs:
          !allowUnfree &&
          isUnfree (lib.lists.toList attrs.meta.license or []) &&
          !allowUnfreePredicate attrs;
  
        # Check whether two sets are mutual exclusive
        mutualExclusive = a: b:
          (builtins.length a) == 0 ||
          (!(builtins.elem (builtins.head a) b) &&
           mutualExclusive (builtins.tail a) b);
  
        # Check whether an package has the license set
        licenseCheckable = attr:
          builtins.hasAttr "meta" attrs && builtins.hasAttr "license" attrs.meta;
  
        # Check whether the license of the package is whitelisted.
        # If the package has no license, print a warning about this and allow the
        # package (return that it is actually whitelisted)
        hasWhitelistedLicense = attrs:
          if licenseCheckable attrs then
            builtins.elem attrs.meta.license whitelistedLicenses
          else
            #builtins.trace "Has no license: ${attrs.name}, allowing installation"
                           true;
  
        # Check whether the license of the package is blacklisted.
        # If the package has no license, print a warning about this and allow the
        # package (return that it is actually not blacklisted)
        hasBlacklistedLicense = attrs:
          if licenseCheckable attrs then
            builtins.elem attrs.meta.license blacklistedLicenses
          else
            #builtins.trace "Has no license: ${attrs.name}, allowing installation"
                           false;
  
      in
      if !(mutualExclusive whitelistedLicenses blacklistedLicenses) then
        throw ''
            Package blacklist (${blacklistedLicenses}) and whitelist
            (${whitelistedLicenses}) are not mutual exclusive.
        ''
      else if hasDeniedUnfreeLicense attrs &&
              !(hasWhitelistedLicense attrs) then
        throwEvalHelp "Unfree" "has an unfree license which is not whitelisted"
      else if hasBlacklistedLicense attrs then
        throwEvalHelp "BlacklistedLicense"
                      "has a license which is blacklisted"
      else if !allowBroken && attrs.meta.broken or false then
        throwEvalHelp "Broken" "is marked as broken"
      else if !allowBroken && attrs.meta.platforms or null != null && !lib.lists.elem result.system attrs.meta.platforms then
  