Define a recursive function SINGLETONS such that if e is any list of numbers and/or symbols then (SINGLETONS e) is a set that consists of all the atoms that occur just once in e. Examples: (SINGLETONS ( ))-> NIL (SINGLETONS '(G A B C B))-> (G A C) SINGLETONS '(H G ABCB))(HGAC) (SINGLETONS (AGABC B)) (G C) SINGLETONS '(B GA BCB))(GAC) [Hint: When e is nonempty, consider the case in which (car e) is a member of (cdr e), and the case in which (car e) is not a member of (edr e).]