code: func (c *Corpus) Merge(other *Corpus) {
  	for i, word := range other.words {
  		freq := other.frequencies[i]
  		if id, ok := c.ids[word]; ok {
  			c.frequencies[id] += freq
  			c.totalFreq += freq
  		} else {
  			id := c.Add(word)
  			c.frequencies[id] += freq - 1
  			c.totalFreq += freq - 1
  		}
  	}
  }
docs: // Merge combines two corpuses. The receiver is the one that is mutated.