code: func (p FieldValues) GroupByShard() map[uint64][]pilosa.FieldValue {
  	m := make(map[uint64][]pilosa.FieldValue)
  	for _, val := range p {
  		shard := val.ColumnID / pilosa.ShardWidth
  		m[shard] = append(m[shard], val)
  	}
  
  	for shard, vals := range m {
  		sort.Sort(FieldValues(vals))
  		m[shard] = vals
  	}
  
  	return m
  }
docs: // GroupByShard returns a map of field values by shard.