code: function(freq){
  		if(freq === undefined)freq = 0;
  		this.phase = (this.phase + freq) % AbstractWave.TWO_PI;
  		if(this.phase < 0){
  			this.phase += AbstractWave.TWO_PI;
  		}
  		return this.phase;
  	}
docs: Ensures phase remains in the 0...TWO_PI interval.
  @param {Number} freq
  normalized progress frequency
  @return {Number} current phase