Note that nil is considered to be a sequence of length zero.
Some operations are useful on both lists and arrays because they deal with ordered sets of elements.
One may ask the number of elements, reverse the ordering, extract a subsequence, and so on. For such purposes Common Lisp provides a set of generic functions on sequences. Some of these operations come in more than one version. In addition, many operations accept one or more optional keyword arguments that can modify the operation in various ways. If the operation requires testing sequence elements according to some criterion, then the criterion may be specified in one of two ways.