public void Set(int index) {
int nBitSlot = Subscript(index);
EnsureLength(nBitSlot + 1);
bits[nBitSlot] |= (uint)1 << (index & bitSlotMask);
}
internal override BitSet Lastpos(int positions) { if (last == null) { last = new BitSet(positions); last.Set(pos); } return(last); }