code: IntIterator keyIterator() {
          return new IntIterator() {
  
              final IntIterator setitr = set.keyIterator();
  
              public boolean hasNext() {
                  return setitr.hasNext();
              }
  
              public int next() {
                  return setitr.next();
              }
  
          };
      }
docs: Returns an iterator over the keys of this map. The keys are returned in
  no particular order.
  
  @return an iterator over the keys of this map.