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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 create_tree(self, tree, base_tree=''):
 """Create a tree on this repository. :param list tree: (required), specifies the tree structure. Format: [{'path': 'path/file', 'mode': :param str base_tree: (optional), SHA1 of the tree you want to update with new data :returns: :class:`Tree <github3.git.Tree>` if successful, else None """

        json = None
        if tree and isinstance(tree, list):
            data = {'tree': tree, 'base_tree': base_tree}
            url = self._build_url('git', 'trees', base_url=self._api)
            json = self._json(self._post(url, data=data), 201)
        return Tree(json) if json else None