class IntSet17a
{
// Constructive Set class, implemented using unordered linked lists
// delete uses stacking

 private Cell list;

 public IntSet17a()
 {
  list = null;
 }

 private IntSet17a(Cell l)
 {
  list = l;
 }

 public IntSet17a delete(int n)
 {
  Cell ptr,list1=null;
  for(ptr=list; ptr!=null&&ptr.first!=n; ptr=ptr.next)
     list1 = new Cell(ptr.first,list1);
  if(ptr==null)
     return this;
  else
     {
      for(ptr=ptr.next; ptr!=null; ptr=ptr.next)
         list1 = new Cell(ptr.first,list1);
      return new IntSet17a(list1);
     }
 }

 public IntSet17a add(int n)
 {
  Cell ptr;
  for(ptr=list; ptr!=null&&ptr.first!=n; ptr=ptr.next) {}
  if(ptr==null)
     {
      Cell list1 = new Cell(n,list);
      return new IntSet17a(list1);
     }
  else
     return this;
 }

 public boolean member(int n)
 {
  Cell ptr;
  for(ptr=list; ptr!=null&&ptr.first!=n; ptr=ptr.next) {}
  return (ptr!=null);
 }

 private static class Cell
 {
  int first;
  Cell next;

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

 public String toString()
 {
  String str="{";
  if(list!=null)
     {
      str+=list.first;
      for(Cell ptr=list.next; ptr!=null; ptr=ptr.next)
          str+=","+ptr.first;
     }
  return str+"}";
 }

}