Base class for [AnimationNode]s that hold one or multiple composite animations. Usually used for [member AnimationTree.tree_root]. [AnimationRootNode] is a base class for [AnimationNode]s that hold a complete animation. A complete animation refers to the output of an [AnimationNodeOutput] in an [AnimationNodeBlendTree] or the output of another [AnimationRootNode]. Used for [member AnimationTree.tree_root] or in other [AnimationRootNode]s. $DOCS_URL/tutorials/animation/animation_tree.html