public static Poly diff (Poly p) throws NullPointerException {
   //EFFECTS: If p is null throws NullPointerException
   //    else returns the Poly obtained by differentiating p.
   Poly q = new Poly ( );
   for (int i = 1; i <= p.degree( ); i++)
      q = q.add(new Poly(p.coeff(i)*i, i - 1));
   return q;
}

public static IntSet getElements (int[ ] a)
      throws NullPointerException {
   //EFFECTS: If a is null throws NullPointerException else returns a set
  //  containing an entry for each distinct element of a.
   IntSet s = new IntSet( );
   for (int i = 0; i < a.length; i++) s.insert(a[i]);
   return s;
}