code: func (s Int64) Difference(s2 Int64) Int64 {
  	result := NewInt64()
  	for key := range s {
  		if !s2.Has(key) {
  			result.Insert(key)
  		}
  	}
  	return result
  }
docs: // Difference returns a set of objects that are not in s2
  // For example:
  // s1 = {a1, a2, a3}
  // s2 = {a1, a2, a4, a5}
  // s1.Difference(s2) = {a3}
  // s2.Difference(s1) = {a4, a5}