func (w *WaterMark) Done(index uint64) {
	w.markCh <- mark{index: index, done: true}
}
func (w *WaterMark) Done(index uint64) {
	w.markCh <- mark{index: index, done: true}
}
func ( w * WaterMark ) Done ( index uint64 ) { w . markCh <- mark { index : index , done : true } }
// Done sets a single index as done.
Done sets a single index as done .