def render pad, lineno, text
      FFI::NCurses.mvwaddstr(pad, lineno, 0, @clearstring) if @clearstring
      FFI::NCurses.mvwaddstr(pad, lineno, 0, text)
    end
def render pad, lineno, text
      FFI::NCurses.mvwaddstr(pad, lineno, 0, @clearstring) if @clearstring
      FFI::NCurses.mvwaddstr(pad, lineno, 0, text)
    end
concrete / derived classes should override this for their specific uses.
 Called if only a row is changed.