Hi, As part of studying Okasaki's PFDS book, I wanted to add Show support for each of the data structures, and some have proven to be challenging, as some of the types are non-regular and automatic derivation of Show doesn't work. I've been able to add some code that introduces a supplementary type class that serves as a way to pass "proof" that the wrapping data type supports traversal for Show-ability, but the solution seems unsatisfactory. I would greatly appreciate any suggestions for improvements. I've attached the code; the relevant bits are in BankersDeque.hs, Example.hs, NestedShowable.hs, and SimpleCatenableDeque.hs. The same code is available here: https://gist.github.com/drvink/30fb2a2b257fc99af281 Thanks, Mark Laws -- |v\ /\ |\ |< |_ /\ \^| //