infix 4 ==
functor FNSET( type Item val == : Item * Item -> bool ) =
struct
    type Item = Item
    type Set  = Item -> bool

    val empty  = fn _ => false
    fun member  s e = s e

    fun insert(e, s) = 
        fn e' => if e == e' then true else s e'

    fun delete(e, []) = 
        fn e' => if e == e' then false else s e' 

    fun union (s, t)     = fn e => s e orelse t e

    fun intersect( s, t) = fn e => s e andalso t e
end;