code: func SampleFunction(function SampleReduceFunc) OptionsFunc {
  	return func(w *Waveform) error {
  		return w.setSampleFunction(function)
  	}
  }
docs: // SampleFunc generates an OptionsFunc which applies the input SampleReduceFunc
  // to an input Waveform struct.
  //
  // This function is used to compute values from audio samples, for use in
  // waveform generation.  The function is applied over a slice of float64
  // audio samples, reducing them to a single value.