code: protected void subscribe( String id, MessagingContext ctx ) throws IOException {
  
  		if( ! canProceed())
  			return;
  
  		Set<MessagingContext> sub = this.routingContext.subscriptions.get( id );
  		if( sub == null ) {
  			sub = new HashSet<> ();
  			this.routingContext.subscriptions.put( id, sub );
  		}
  
  		sub.add( ctx );
  	}
docs: Registers a subscription between an ID and a context.
  @param id a client ID
  @param ctx a messaging context
  @throws IOException