def setDomainOnActivation(domain, event):
    """Set the permission id upon registration activation.

    Let's see how this notifier can be used. First we need to create an event
    using the permission instance and a registration stub:

    >>> class Registration:
    ...     def __init__(self, obj, name):
    ...         self.component = obj
    ...         self.name = name

    >>> domain1 = TranslationDomain()
    >>> domain1.domain
    '<domain not activated>'

    >>> import zope.interface.interfaces
    >>> event = zope.interface.interfaces.Registered(
    ...     Registration(domain1, 'domain1'))

    Now we pass the event into this function, and the id of the domain should
    be set to 'domain1'.

    >>> setDomainOnActivation(domain1, event)
    >>> domain1.domain
    'domain1'
    """
    domain.domain = event.object.name
