Just tripped over this: Adjoint Folds and Unfolds Or: Scything through the Thicket of Morphisms Folds and unfolds are at the heart of the algebra of programming. They allow the cognoscenti to derive and manipulate programs rigorously... Ralf Hinze Lecture Notes in Computer Science, 2010, Volume 6120, Mathematics of Program Construction, Pages 195-228 -- -- Regards, KC