It seems to me that worlds permit sharing immutable data structures, and that Warth & Kay's operations on worlds do not permit sharing mutations done in paralle

It seems to me that worlds permit sharing immutable data structures, and that Warth & Kay's operations on worlds do not permit sharing mutations done in parallel worlds. The Worlds paper you cited [http://www.vpri.org/pdf/rn2008001_worlds.pdf http://www.vpri.org/pdf/rn2008001_worlds.pdf] cites the COP overview paper [http://p-cos.net/documents/contextl-overview.pdf http://p-cos.net/documents/contextl-overview.pdf] mentioned near the top of the link jrockway gave. Warth & Kay say "Similarly, in order to support context-oriented programming (COP) [8], we may want to add a third key to our lookup table that identifies a context." So that gives me some confidence in my interpretation.