code: func (c *MockStore) Add(cwr *ChunkWriteRequest) {
  	if !c.Drop {
  		intervalHint := cwr.Key.Archive.Span()
  		itgen, err := chunk.NewIterGen(cwr.Chunk.Series.T0, intervalHint, cwr.Chunk.Encode(cwr.Span))
  		if err != nil {
  			panic(err)
  		}
  		c.results[cwr.Key] = append(c.results[cwr.Key], itgen)
  		c.items++
  	}
  }
docs: // Add adds a chunk to the store