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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 write_tree(self):
 """Writes this index to a corresponding Tree object into the repository's object database and return it. :return: Tree object representing this index :note: The tree will be written even if one or more objects the tree refers to does not yet exist in the object database. This could happen if you added Entries to the index directly. :raise ValueError: if there are no entries in the cache :raise UnmergedEntriesError: """

        # we obtain no lock as we just flush our contents to disk as tree
        # If we are a new index, the entries access will load our data accordingly
        mdb = MemoryDB()
        entries = self._entries_sorted()
        binsha, tree_items = write_tree_from_cache(entries, mdb, slice(0, len(entries)))

        # copy changed trees only
        mdb.stream_copy(mdb.sha_iter(), self.repo.odb)

        # note: additional deserialization could be saved if write_tree_from_cache
        # would return sorted tree entries
        root_tree = Tree(self.repo, binsha, path='')
        root_tree._cache = tree_items
        return root_tree