code: def iterate_symbols():
      
      for prefix in search_prefixes():
          package = importlib.import_module(prefix)
          for importer, modname, ispkg in pkgutil.walk_packages(path=package.__path__, onerror=lambda x: None):
              network = network_for_netcode(modname)
              if network:
                  yield network.symbol.upper()
docs: Return an iterator yielding registered netcodes.