code: def top_level(self):
          
          top = list(self.cell_dict.values())
          for cell in self.cell_dict.values():
              for dependency in cell.get_dependencies():
                  if dependency in top:
                      top.remove(dependency)
          return top
docs: Output the top level cells from the GDSII data.
  
          Top level cells are those that are not referenced by any other
          cells.
  
          Returns
          -------
          out : list
              List of top level cells.