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