src/vs/workbench/contrib/notebook/browser/notebookKernelServiceImpl.ts/NotebookKernelService/updateKernelNotebookAffinity
class NotebookKernelService:	updateKernelNotebookAffinity(kernel: INotebookKernel, notebook: URI, preference: number | undefined): void {
		const info = this._kernels.get(kernel.id);
		if (!info) {
			throw new Error(`UNKNOWN kernel '${kernel.id}'`);
		}
		if (preference === undefined) {
			info.notebookPriorities.delete(notebook);
		} else {
			info.notebookPriorities.set(notebook, preference);
		}
		this._onDidChangeNotebookAffinity.fire();
	}