code: def step(self):
          
          if self.index >= len(self.tasklist):
              raise TaskError("step(): sequential compound task %s finished" % self)
  
          self.check_state()
  
          # Select next task from the set and advance the index
          self.task = self.tasklist[self.index]
          self.index += 1
  
          return self.runTask(self.task)
docs: Run the next child task and wait for completion (no timeout).