func (w *WaterMark) BeginMany(indices []uint64) {
	atomic.StoreUint64(&w.lastIndex, indices[len(indices)-1])
	w.markCh <- mark{index: 0, indices: indices, done: false}
}
func (w *WaterMark) BeginMany(indices []uint64) {
	atomic.StoreUint64(&w.lastIndex, indices[len(indices)-1])
	w.markCh <- mark{index: 0, indices: indices, done: false}
}
// BeginMany works like Begin but accepts multiple indices.