code: func (q *MatchQuery) CutoffFrequency(cutoff float64) *MatchQuery {
  	q.cutoffFrequency = &cutoff
  	return q
  }
docs: // CutoffFrequency can be a value in [0..1] (or an absolute number >=1).
  // It represents the maximum treshold of a terms document frequency to be
  // considered a low frequency term.