if (selected.has(n)) {
      unselect(n);
    } else {
      select(n);
    }
    updateButtons();
  }

  function unselect(n) {
    if (setNodeHighlight(n, false)) selected.delete(n);
  }

  function select(n, elem) {
    if (setNodeHighlight(n, true)) selected.set(n, true);
  }

  function nodeId(elem) {
    const id = elem.id;
    if (!id) return -1;
    if (!id.startsWith('node')) return -1;