member
(member element list)
The member function searches the given list for an element and returns the remaining portion of the list.
The function returns nil, if element is not a member of list.
: (member '1 '(1 2 3 4 ))
(1 2 3 4)
: (member 'c '(a b c a b c))
(C A B C)
: (member '1 '(a b c))
nil