<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_subassistant_tree(self):
 """Returns a tree-like structure representing the assistant hierarchy going down from this assistant to leaf assistants. For example: [(<This Assistant>, )] Returns: a tree-like structure (see above) representing assistant hierarchy going down from this assistant to leaf assistants """

        if '_tree' not in dir(self):
            subassistant_tree = []
            subassistants = self.get_subassistants()
            for subassistant in subassistants:
                subassistant_tree.append(subassistant.get_subassistant_tree())
            self._tree = (self, subassistant_tree)
        return self._tree