Hi guys. We are doing graph traversal (for Fuel) and we would love a collection to be really fast and only support two simple operations: #add: and #includes: The idea is that each object we process of the graph we put it in the collection and then for each object we have to ask whether it is included or not (to avoid cycles and putting the same object more than once). VW seems to have ObjectRegistry (subclass of IdentitySet) which is more or less that. It is used by ObjectTracer, which in fact is what we want to do. And even more, Parcels uses that ;) (or an extension of that). So...I wonder, is there a special collection implementation ideal for this case? is something similar to ObjectRegistry available for pharo ? Thanks in advance, -- Mariano http://marianopeck.wordpress.com