[Pharo-project] ReferenceStream, tree or graph?

Hi folks. I have a question. Reading http://wiki.squeak.org/squeak/2318 it says: "This is a way of serializing a tree of objects into disk file. A ReferenceStream can store one or more objects in a persistent form, including sharing and cycles." I am confused. If it has cycles, shouldn't it be a graph instead of a tree? I know this is subtle, but it is important for me. I want to know if ReferenceStream supports a graph of objects, NOT only a tree. Does someone know? Thanks Mariano