code: def do_trace(self, arg):
          
          if arg:     # XXX this check is to be removed
              raise CmdError("too many arguments")
          if self.lastEvent is None:
              raise CmdError("no current thread set")
          self.lastEvent.get_thread().set_tf()
          return True
docs: t - trace at the current assembly instruction
          trace - trace at the current assembly instruction