structure AbsIntSet : SetSig =
struct
    type Item = int
    abstype Set  = Set of Item list
    with
        val empty = Set []

        fun isEmpty (Set []) = true 
          | isEmpty  _       = false

        fun member (Set [])      e  = false
          | member (Set(h :: t)) e  = (e = h) orelse member (Set t) e;

        fun insert(e, s as Set xs)       = if member s e then s 
                                           else Set(e :: xs);

        fun delete(e, Set [])       = [] 
          | delete(e, Set(h::t))    = if e = h then Set t 
                                      else insert(h, delete(e, Set t));

        fun union(Set []   , s)     = s
          | union(Set(h::t), s)     = insert(h, union(Set t, s));

        fun intersect(Set [],    s) = Set []
          | intersect(Set(h::t), s) = 
              if member s h then insert(h, intersect(Set t, s)
                            else           intersect(Set t, s);
    end
end;