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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 collect_publications(self):
 """ Recursively collect list of all publications referenced in this tree and all sub-trees. Returns: list: List of UUID strings. """

        pubs = list(self.sub_publications)

        for sub_tree in self.sub_trees:
            pubs.extend(sub_tree.collect_publications())

        return pubs