anchor.diff:          , createHome ? false
           , useDefaultShell ? false
           , password ? null
  +        , isSystemUser ? true
           }:
  -        { inherit name description uid group extraGroups home shell createHome password; };
  +        { inherit name description uid group extraGroups home shell createHome password isSystemUser; };
   
       in map addAttrs (defaultUsers ++ config.users.extraUsers);
   
  
positive.code:             read shell
              read createHome
              read password
  
              if ! curEnt=$(getent passwd "$name"); then
                  useradd --system \
                      --comment "$description" \
                      ''${uid:+--uid $uid} \
                      --gid "$group" \
  
negative.code:           }
          ];
  
        # !!! Use NixOS module system to add default attributes.
        addAttrs =
          { name
          , description
  