 topic's publishers 
 are retained until they are acknowledged by this durable 
 subscriber or they have expired.

 <P>Sessions with durable subscribers must always provide the same client 
 identifier. In addition, each client must specify a name that uniquely 
 identifies (within client identifier) each durable subscription it creates.
 Only one session at a time can have a <code>TopicSubscriber</code> for a 
 particular durable subscription. 

 <P>A client can change an existing durable subscription by c