class IntSet19
{
// Constructive Set class, 
// implemented using unordered ADT Lists

 private List list;

 public IntSet19()
 {
  list = List.empty();
 }

 private IntSet19(List l)
 {
  list = l;
 }

 public IntSet19 delete(int n)
 {
  List list1 = delete(n,list);
  return new IntSet19(list1);
 }

 private static List delete(int n,List l)
 {
  if(l.isEmpty())
     return List.empty();
  else if(l.head()==n)
     return l.tail();
  else
     return delete(n,l.tail()).cons(l.head());
 }

 public IntSet19 add(int n)
 {
  List ptr;
  for(ptr=list; !ptr.isEmpty()&&ptr.head()!=n; ptr=ptr.tail());
  if(ptr.isEmpty())
     {
      List list1 = list.cons(n);
      return new IntSet19(list1);
     }
  else
     return this;
 }

 public boolean member(int n)
 {
  List ptr;
  for(ptr=list; !ptr.isEmpty()&&ptr.head()!=n; ptr=ptr.tail());
  return (!ptr.isEmpty());
 }

 public String toString()
 {
  String str="{";
  if(!list.isEmpty())
     {
      str+=list.head();
      for(List ptr=list.tail(); !ptr.isEmpty(); ptr=ptr.tail())
          str+=","+ptr.head();
     }
  return str+"}";
 }

}