code: func (c *Corpus) IDFreq(id int) int {
  	size := atomic.LoadInt64(&c.maxid)
  	maxid := int(size)
  
  	if id >= maxid {
  		return 0
  	}
  	return c.frequencies[id]
  }
docs: // IDFreq returns the frequency of a word given an ID. If the word isn't in the corpus it returns 0.