def s(self):
		"""
		step to the next node in the execution order
		"""
		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))
step to the next node in the execution order