def cmd_stru(param)
      send_param_required and return if param.nil?
      send_unauthorised and return unless logged_in?
      if param.upcase.eql?("F")
        send_response "200 OK"
      else
        send_response "504 STRU is an obsolete command"
      end
    end
def cmd_stru(param)
      send_param_required and return if param.nil?
      send_unauthorised and return unless logged_in?
      if param.upcase.eql?("F")
        send_response "200 OK"
      else
        send_response "504 STRU is an obsolete command"
      end
    end
like the MODE and TYPE commands, stru[cture] dates back to a time when the FTP
 protocol was more aware of the content of the files it was transferring, and
 would sometimes be expected to translate things like EOL markers on the fly.

 These days files are sent unmodified, and F(ile) mode is the only one we
 really need to support.