def _propagate_packages(self):
        r"""Propogate packages.

        Make sure that all the packages included in the previous containers
        are part of the full list of packages.
        """

        super()._propagate_packages()

        for item in (self.preamble):
            if isinstance(item, LatexObject):
                if isinstance(item, Container):
                    item._propagate_packages()
                for p in item.packages:
                    self.packages.add(p)