Interface implemented by JACOB objects that are to be indexed. Indexed objects have the nice feature that they
can be retrieved from the execution queue by their index key. This is handy for introspecting the state of
the execution queue. Note that indexed objects are made available so long as they are referenced in some way by
objects in the queue. The reference does not need to be direct; e.g. if a
ChannelListener
in the execution queue references an indexed object, that indexed object will be indexed.