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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def relation_id(relation_name=None, service_or_unit=None):
 """The relation ID for the current or a specified relation"""

    if not relation_name and not service_or_unit:
        return os.environ.get('JUJU_RELATION_ID', None)
    elif relation_name and service_or_unit:
        service_name = service_or_unit.split('/')[0]
        for relid in relation_ids(relation_name):
            remote_service = remote_service_name(relid)
            if remote_service == service_name:
                return relid
    else:
        raise ValueError('Must specify neither or both of relation_name and service_or_unit')