code: def _propagate_packages(self):
          r
  
          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)
docs: r"""Propogate packages.
  
          Make sure that all the packages included in the previous containers
          are part of the full list of packages.