Author here. I double checked the Hyperspec [1] and you're quite right. I'll fix the article tomorrow, thank you for noticing![1] http://www.lispworks.com/documentation/lw60/CLHS/Body/f_redu... fold is a natural consequence of church lists, probably the most fundamental way of expressing the list abstraction: \f \x (f a0 (f a1 (f a2 x))) So fold is just applying a list (function) to 2 arguments. O