getPrevChildOffset = (timeline, timePosition) => {
  if (stringStartsWith(timePosition, '<')) {
    const goToPrevAnimationOffset = timePosition[1] === '<';
    const prevAnimation = /** @type {Tickable} */(timeline._tail);
    const prevOffset = prevAnimation ? prevAnimation._offset + prevAnimation._delay : 0;
    return goToPrevAnimationOffset ? prevOffset : prevOffset + prevAnimation.duration;
  }
}
Timeline's children offsets positions parser
@param  {Timeline} timeline
@param  {String} timePosition
@return {Number}