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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 arbiter(**params):
    '''Obtain the ``arbiter``.

    It returns the arbiter instance only if we are on the arbiter
    context domain, otherwise it returns nothing.
    '''
    arbiter = get_actor()
    if arbiter is None:
        # Create the arbiter
        return set_actor(_spawn_actor('arbiter', None, **params))
    elif arbiter.is_arbiter():
        return arbiter