def alias_activity(self, activity_id=None, alias_id=None):
raise Unimplemented()<EOL>
Adds an Id to an Activity for the purpose of creating
        compatibility.
        The primary Id of the Activity is determined by the provider.
        The new Id performs as an alias to the primary Id. If the alias
        is a pointer to another activity, it is reassigned to the given
        activity Id.
        arg:    aliasId (osid.id.Id): the alias Id
        raise:  ALREADY_EXISTS - aliasId is already assigned
        raise:  NotFound - activityId not found
        raise:  NullArgument - activityId or aliasId is null
        raise:  OperationFailed - unable to complete request
        raise:  PermissionDenied - authorization failure
        compliance: mandatory - This method must be implemented.