NOT11 — subscription_change issue

Summary: The text describing the behaviour of implementations calling
subscription_change on their suppliers:

Suppliers that are not interested in the event types currently
subscribed to will not invoke obtain_subscription_types, and will
raise the NO_IMPLEMENT exception in their implementation of the
subscription_change operation.

The mechanism (raising NO_IMPLEMENT) is crude, and the continuing
behaviour of the channel is unspecified; i.e. should the channel
always call subscription_change and just put up with the exception? I
should think not. However, should the channel assume that this
supplier will never be interested in subscriptions ever again? Perhaps
not. What if the supplier raises NO_IMPLEMENT, and then later calls
obtain_subscription_types - does this indicate that it is now
interested in subscription changes?