anchor.diff:    AudioContext _context;
     
     double frequencyCutoff = 25000.0;
  -  double prevFrequencyCutoff = 0.0;
  +  double _prevFrequencyCutoff = 0.0;
     // Higher values lead to more distortion.
     double _qResonance = 0.001;
     double _prevQResonance = 0.0;
  
positive.code:     _filter.frequency.setValueAtTime(frequencyCutoff, now);
      _filter.frequency.exponentialRampToValueAtTime(rampTo, now + 1.0);
      
      prevFrequencyCutoff = frequencyCutoff;
      _prevQResonance = _qResonance;
    }
  }
negative.code: 
      bool valueChanged = false;
      
      if (frequencyCutoff != _prevFrequencyCutoff) {
        _filter.frequency.cancelScheduledValues(now);
        _filter.frequency.setValueAtTime(frequencyCutoff, now);
        valueChanged = true;
  