The chibi "ephemerons" are named such as a reminder to myself to actually implement proper ephemerons, but the name is a lie. They are currently simply key-weak pairs.

An excellent discussion of the implementation details was made by Bruno Haible:

http://users.csc.calpoly.edu/~gfisher/work/specl/documentation/related-work/formal-semantics/WeakDatastructures-writeup.html

Alex