add(num) {
    var add = new Add(num);
    var thisChain = this.mathOps.length;
    var nextChain = this.output;
    return p5.prototype._mathChain(this, add, thisChain, nextChain, Add);
  }
Add a value to the p5.Oscillator's output amplitude,
 and return the oscillator. Calling this method
 again will override the initial add() with new values.

 @method  add
 @for p5.Envelope
 @param {Number} number Constant number to add
 @return {p5.Envelope} Envelope Returns this envelope
                                    with scaled output