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Fold is one of the most commonly used catamorphisms in functional languages. Erlang has foldl and foldr, which are used very frequently to encapsulate some of the very common patterns of computation as higher order operators instead of using recursion directly. Fold's dual is unfold (anamorphism), which unfortunately does not enjoy an equal popularity amongst functional programmers. While fold is a recursion operator that consumes a collection, it's dual unfold encapsulates a common pattern that produces streams / collections from a single object.