Two qustions about the latest BitMaps library

Hi there, this sentence seems to suggest to use BitMaps, the keys are sequential. I don’t think it is necessary, is it?

Besides, is it possible to return the count of trues from this library efficiently? Just like the length function in the EnumerableSet library.

Thanks.

Answering my own second question: a Counters.Counter can be added in a smart contract calling set and unset functions. With proper setups, the counter value can work as length of the Bitmap.