src/vs/workbench/contrib/notebook/common/notebookEditorInput.ts/isCompositeNotebookEditorInput
export function isCompositeNotebookEditorInput(thing: unknown): thing is ICompositeNotebookEditorInput {
	return !!thing
		&& typeof thing === 'object'
		&& Array.isArray((<ICompositeNotebookEditorInput>thing).editorInputs)
		&& ((<ICompositeNotebookEditorInput>thing).editorInputs.every(input => input instanceof NotebookEditorInput));
}