How can I use a type with generic arguments as a constraint?

class KeyFrame<T>
{
    public float Time;
    public T Value;
}

// I want any kind of Keyframe to be accepted
class Timeline<T> where T : Keyframe<*>
{
}

class Timeline<TKeyframe, TKeyframeValue>
     where TKeyframe : Keyframe<TKeyframeValue>,
{
}

public abstract class FooBase
{
  private FooBase() {} // Not inheritable by anyone else
  public class Foo<U> : FooBase {...generic stuff ...}

  ... nongeneric stuff ...
}

public class Bar<T> where T: FooBase { ... }
...
new Bar<FooBase.Foo<string>>()

class TimeLine<T>
{
private IList<KeyFrame<T>> keyFrameList;
...
}

class Timeline<T>
{
  List<KeyFrame<T>> _frames = new List<KeyFrame<T>>(); //Or whatever...

  ...
}

class KeyFrame<T> {
  public float Time;
  public T Value;

  class Timeline<U> where U : Keyframe<T> {
  }
}