anchor.diff:  
   /// An animation that tracks the minimum of two other animations.
   ///
  -/// The [value] of this animation is the maximum of the values of 
  +/// The [value] of this animation is the maximum of the values of
   /// [first] and [next].
   class AnimationMin<T extends num> extends CompoundAnimation<T> {
  +  /// Creates an [AnimationMin].
  +  ///
  +  /// Both arguments must be non-null. Either can be an [AnimationMin] itself
  +  /// to combine multiple animations.
     AnimationMin(Animation<T> first, Animation<T> next): super(first: first, next: next);
   
     @override
  
positive.code:   }
  
    /// Gets the status of this animation based on the [first] and [next] status.
    /// 
    /// The default is that if the [next] animation is moving, use its status.
    /// Otherwise, default to [first].
    @override
  
negative.code: ///
  /// For example, to create an animation that is the sum of two others, subclass
  /// this class and define `T get value = first.value + second.value;`
  ///
  /// By default, the [status] of a [CompoundAnimation] is the status of the
  /// [next] animation if [next] is moving, and the status of the [first]
  /// animation otherwise.
  abstract class CompoundAnimation<T> extends Animation<T>
    with AnimationLazyListenerMixin, AnimationLocalListenersMixin, AnimationLocalStatusListenersMixin {
  