code: func (h *Handle) SetAny(serial bool) (uint64, error) {
  	if h.Unselected() == 0 {
  		return invalidPos, ErrNoBitAvailable
  	}
  	return h.set(0, 0, h.bits-1, true, false, serial)
  }
docs: // SetAny atomically sets the first unset bit in the sequence and returns the corresponding ordinal