def __compact_notification_addresses(addresses_):
result = {}<EOL>addresses = dict(addresses_)<EOL>default = addresses.pop('<STR_LIT:default>', None)<EOL>for key, value in __reverse_group_dict(addresses).items():<EOL><INDENT>result['<STR_LIT:U+002C>'.join(value)] = key<EOL><DEDENT>if default:<EOL><INDENT>result['<STR_LIT:default>'] = default<EOL><DEDENT>return result<EOL>
Input::

    {
        'default': 'http://domain/notice',
        'event1': 'http://domain/notice',
        'event2': 'http://domain/notice',
        'event3': 'http://domain/notice3',
        'event4': 'http://domain/notice3'
    }

Output::

    {
        'default': 'http://domain/notice',
        'event1,event2': 'http://domain/notice',
        'event3,event4': 'http://domain/notice3'
    }