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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 mk_dropdown_tree(cls, model, for_node=None):
 """ Creates a tree-like list of choices """


        options = [(0, _('-- root --'))]
        for node in model.get_root_nodes():
            cls.add_subtree(for_node, node, options)
        return options