Most programming languages support collections, represented by an in-memory data structure, a file, a database, or a generating function. A programming language system gives us typically one of the two interfaces to systematically access elements of a collection. One traversal API is based on enumerators -- e.g., for-each, map, filter higher-order procedures -- of which the most general is fold. The second approach relies on streams, a.k.a. cursors, lazy lists.