return new Iterator<V>()
        {
            int cur = -1;
            V next = computeNext();
            @Override
            public boolean hasNext()
            {
                return next != null;
            }

            private V computeNext() {
            	V ret = null;
            	do {
                	cur = availabilityMap.nextSetBit(cur + 1);
                	ret = ids.get(cur);
                } while (ret == null && cur != -1);
            	return ret;
            }

            @Override
            public V next()
            {
                V ret = next;
                next = computeNext();
                return ret;
            }
            //TODO add remove support?
        };