Node containing the overlay content. It must be a Node instance that implements the Overlayable interface. Implementations should reject (through an AssertError) any objects that do not inherit from Node.
Type of the overlay node, as a fallback sequence. This information that can be used to guide the overlay system by identifying the overlay's purpose. See OverlayIO.Types and OverlayIO.types for commonly used types.
The system should choose the first type from the list that it supports. If none of the types in the list are recognized, OverlayType.init should be assumed. All overlay systems must support OverlayType.init as a general purpose overlay.
addChildOverlay
Insert new overlay content.
Overlay content should remain alive until the node specifies a toRemove status. Overlays should not be added if they're not already in the node tree.