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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 expandDescendants(self, branches):
 """ Expand descendants from list of branches :param list branches: list of immediate children as TreeOfContents objs :return: list of all descendants """

        return sum([b.descendants() for b in branches], []) + \
            [b.source for b in branches]