public Animation WithConcurrent(Action<double> callback, double start = 0.0f, double end = 1.0f, Func<double, double> easing = null, double beginAt = 0.0f, double finishAt = 1.0f) { Animation child = new Animation (callback, start, end, easing); child.beginAt = beginAt; child.finishAt = finishAt; children.Add (child); return this; }