We can get folds and unfolds over algebraic datatypes for free. (But who in
their right mind would want to jump through all the hoops? (Co-)recursion
schemes should really be in the language out of the box.)

See also:

Philip Wadler, Recursive types for free!

Erik Meijer, Maarten Fokkinga, Ross Patterson, Functional Programming with
Bananas, Lenses, Envelopes and Barbed Wire

Conor McBride, Clowns to the Left of me, Jokers to the Right: Dissecting
Data Structures

Richard Bird, Oege de Moor, The Algebra of Programming

Fritz Ruehr, The Evolution of a Haskell Programmer

The ideas are scattered all over the place, but this implementation is
based on the code from McBride's Clowns and Jokers, where it's but a
stepping stone towards building something else entirely.