def abort
      line = "ABOR" + CRLF
      print "put: ABOR\n" if @debug_mode
      @sock.send(line, Socket::MSG_OOB)
      resp = getmultiline
      unless ["426", "226", "225"].include?(resp[0, 3])
        raise FTPProtoError, resp
      end
      return resp
    end
Aborts the previous command (ABOR command).