<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def iterate_symbols():
 """ Return an iterator yielding registered netcodes. """

    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()