public synchronized void evaluateForCallbackCompletion( @Nonnull Packet stanza )
        {
            Log.trace( "Evaluating stanza for callback completion..." );
            if ( stanza.getElement().element(FMUC) == null ) {
                throw new IllegalArgumentException( "Argument 'stanza' must have an FMUC child element." );
            }

            // Ignore if we do not have a callback waiting for this stanza.
            final Iterator<PendingCallback> iter = pendingEcho.iterator();
            while (iter.hasNext()) {
                final PendingCallback item = iter.next();
                if ( item.isMatch(stanza) ) {
                    Log.trace( "Invoking callback, as peer '{}' echo'd back stanza: {}", getPeer(), stanza.toXML() );
                    item.complete();
                    iter.remove();
                }
            }
            Log.trace( "Finished evaluating stanza for callback completion." );
        }
A list of stanzas that need to have been echo'd by a remote FMUC node, before they can be processed locally.
This collection _does not_ include stanzas needed to set up the initial join. This collection is only used
for subsequent stanzas that are shared in a setting where echo-ing is required (due to the mode of the
federation being defined as 'master-slave').
xmppserver/src/main/java/org/jivesoftware/openfire/muc/spi/FMUCHandler.java