code: func (r *Row) segment(shard uint64) *rowSegment {
  	if i := sort.Search(len(r.segments), func(i int) bool {
  		return r.segments[i].shard >= shard
  	}); i < len(r.segments) && r.segments[i].shard == shard {
  		return &r.segments[i]
  	}
  	return nil
  }
docs: // segment returns a segment for a given shard.
  // Returns nil if segment does not exist.