A pair structure which is either or which has a first element and which has rest another List. The first elements of the List and its transitive non‐Nil rest Lists form the elements of an R·D·F collection.

The description above is more restrictive than the formal ontological definition for Lists, which places no constraints on the number (or presence) of its first and rest properties. Conformance to these expectations is strongly recommended, but implementations should be aware that some data may take a more flexible approach.