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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def print_tree(self, tree=None):
 """ Print referrers tree to console. keyword arguments tree -- if not None, the passed tree will be printed. Otherwise it is based on the rootobject. """

        if tree is None:
            self._print(self.root, '', '')
        else:
            self._print(tree, '', '')