code: public IntIterator unsetIntIterator() {
        return new IntIterator() {
            @Override
            public boolean hasNext() {
                return this.i >= 0;
            }
  
            @Override
            public int next() {
                this.j = this.i;
                this.i = ImmutableBitSet.this.nextUnsetBit(this.i + 1);
                return this.j;
            }
  
            private int i = ImmutableBitSet.this.nextUnsetBit(0);
  
            private int j;
        };
    }
docs: Iterate over the unset bits
  
  @return an iterator