anchor.diff:  
           for f in $authKeyFiles; do
             if [ -f "$f" ]; then
  -            authKeys="$(${pkgs.coreutils}/bin/cat "$f"),$authKeys"
  +            authKeys="$(${pkgs.coreutils}/bin/cat "$f") ${marker1},$authKeys"
             fi
           done
   
  
positive.code: 
    mkAuthkeyScript =
      let
        marker1 = "### NixOS will regenerate this line and every line below it.";
        marker2 = "### NixOS will regenerate this file. Do not edit!";
        users = map (userName: getAttr userName config.users.extraUsers) (attrNames config.users.extraUsers);
        usersWithKeys = flip filter users (u:
  
negative.code:             rm -f "$authfile"
              authKeys="${marker2},$authKeys"
            else
              ${pkgs.gnused}/bin/sed -i '/${marker1}/ d' "$authfile"
            fi
            for key in $authKeys; do ${pkgs.coreutils}/bin/echo "$key" >> "$authfile"; done
          fi
  