code: func SetWarp(warp float64) Option {
  	return func(ngram *NGramIndex) error {
  		if warp < 0.0 || warp > 1.0 {
  			return errors.New("bad 'warp' value for n-gram index")
  		}
  		ngram.warp = warp
  		return nil
  	}
  }
docs: // SetWarp must be used to pass warp to NGramIndex c-tor