<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def _reroot(self):
        '''Run the re-rooting algorithm in the Rerooter class.'''
        rerooter = Rerooter()
        self.tree = rerooter.reroot_by_tree(self.reference_tree, 
                                                self.tree)