and returns the second.

:
:
:

A linked list can either be NIL, representing the empty list, or a of an element (so-called head) and a smaller list (tail). The predicate returns for the value , and for a non-empty list:

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Alternatively, with , the construct obviates the need for an explicit NULL test:

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:

As an example of the use of pairs, the shift-and-increment function that maps to can be defined as
:
: which allows us to give perhaps the most

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