class IntSet15
{
// Destructive Set class, sets stored as ordered linked lists
// with dummy first element. Uses recursive methods.

 private Cell list;

 public IntSet15()
 {
  list = new Cell(0,null);
 }

 public void delete(int n)
 {
  delete(n,list);
 }

 private static void delete(int n,Cell l)
 {
  if(l.next!=null)
     if(l.next.first==n)
        l.next=l.next.next;
     else if(l.next.first<n)
        delete(n,l.next);
 }

 public void add(int n)
 {
  add(n,list);
 }

 private static void add(int n,Cell l)
 {
  if(l.next==null||n<l.next.first)
     l.next = new Cell(n,l.next);
  else if(n>l.next.first)
     add(n,l.next);
 }

 public boolean member(int n)
 {
  return member(n,list.next);
 }
 
 private static boolean member(int n,Cell l)
 {
  if(l==null||n<l.first)
     return false;
  else if(n==l.first)
     return true;
  else
     return member(n,l.next);
 }

 public String toString()
 {
  if(list.next==null)
     return "{}";
  else
     return "{"+toString(list.next)+"}";
 }

 private static String toString(Cell l)
 {
  if(l.next==null)
     return ""+l.first;
  else
     return l.first+","+toString(l.next);
 }

 private static class Cell
 {
  int first;
  Cell next;
  
  Cell(int f,Cell n)
  {
   first=f;
   next=n;
  }
 }
}