_updateAfter(time, value) {
    if (this._voicesInUse._searchAfter(time) === null) {
      return;
    } else {
      this._voicesInUse._searchAfter(time).value += value;
      var nextTime = this._voicesInUse._searchAfter(time).time;
      this._updateAfter(nextTime, value);
    }
  }
Private method to ensure accurate values of this._voicesInUse
Any time a new value is scheduled, it is necessary to increment all subsequent
scheduledValues after attack, and decrement all subsequent
scheduledValues after release

@private
@for p5.PolySynth
@param  {[type]} time  [description]
@param  {[type]} value [description]
@return {[type]}       [description]