code: def s(self):
  		
  		next_node=self._exe_order[self.step]
  		self._eval(next_node)
  		self.step+=1
  		if self.step==len(self._exe_order):
  			return self._finish()
  		else:
  			# if stepping, return the value of the node we just
  			# evaled
  			return self._break(value=self._cache.get(next_node.name))
docs: step to the next node in the execution order