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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 regroup(self, group):
 """Deprecated method to apply new group to items. Equivalent functionality possible using: ViewableTree(tree.relabel(group='Group').values()) """

        if util.config.future_deprecations:
            self.param.warning('%s.regroup is deprecated, use relabel '
                               'method with a group argument instead.'
                               % type(self).__name__)
        new_items = [el.relabel(group=group) for el in self.data.values()]
        return reduce(lambda x,y: x+y, new_items)