code: func SetN(n int) Option {
  	return func(ngram *NGramIndex) error {
  		if n < 2 || n > maxN {
  			return errors.New("bad 'n' value for n-gram index")
  		}
  		ngram.n = n
  		return nil
  	}
  }
docs: // SetN must be used to pass N (gram size) to NGramIndex c-tor