A linked list can be used as a representation of a set. member (written &isin) tests whether a given item is an element of the list. member returns the remainder of the list beginning with the desired element, although usually member is used as a predicate to test whether the element is present or not.
(member 'dick '(tom dick harry)) -> (dick harry) (member 'fred '(tom dick harry)) -> nil
(defun member (item lst)
(if (null lst)
nil
(if (eql item (first lst))
lst
(member item (rest lst)) ) ) )
public static Cons member (Object item, Cons lst) {
if ( lst == null )
return null;
else if ( item.equals(first(lst)) )
return lst;
else return member(item, rest(lst)); }