code: void markMerged() {
          Exceptions.checkNotClosed(this.closed, this);
          Preconditions.checkState(!this.merged, "StreamSegmentReadIndex %d is already merged.", this.metadata.getId());
          log.debug("{}: Merged.", this.traceObjectId);
          this.merged = true;
      }
docs: Marks this Read Index as merged into another one.