This snippet is really getting held back by the Bitmapset, both interface (eg. the del_members/add_members dance) and the structure (eg. with the double indirec

This snippet is really getting held back by the Bitmapset, both interface (eg. the del_members/add_members dance) and the structure (eg. with the double indirection, and unnecessary implicit checks for reallocation). You don't really want to rewrite the bit indexing, but it feels like you're paying a lot of cruft just to avoid a few “x / 8”.