A set of pending HRMP close channel requests that are going to be closed during the session
change. Used for checking if a given channel is registered for closure.
The set is accompanied by a list for iteration.
Invariant:
There are no channels that exists in list but not in the set and vice versa.
A set of pending HRMP close channel requests that are going to be closed during the session change. Used for checking if a given channel is registered for closure.
The set is accompanied by a list for iteration.
Invariant: