import std.algorithm.comparison : equal;

auto b = BitArray([1, 0, 1, 0, 1, 1]);

b[] = true;
// all bits are set
assert(b.bitsSet.equal([0, 1, 2, 3, 4, 5]));

b[] = false;
// none of the bits are set
assert(b.bitsSet.empty);