function Timeline (tweens) {
  this.tweens = tweens
  this.index = 0
}
Timeline.prototype.advance = function(){
  var tl = this.tweens[this.index % this.tweens.length]
  if (tl.index == -1) {
    tl.reset()
    tl.index = 0
    requestAnimationFrame(this.advance.bind(this))
  }
  else if (tl.index < tl.timeline.length){
    tl.timeline[ tl.index ](this.advance.bind(this))
    tl.index += 1
  }
  else {
    this.index += 1
    tl.index = -1
    requestAnimationFrame(this.advance.bind(this))
  }
}