code: function makeChildrenObserver(e) {
    function add    (v)                   { e.appendChild(v); }
    function _delete(v, i, o, {oldValue}) { e.removeChild(oldValue); }
    function update (v, i, c, {oldValue}) {
      if (i !== 'length') { e.replaceChild(v, oldValue); }
    }
    return makeObserver({add, update, delete: _delete});
  }
docs: Make observers for children, attributes, and subattributes. -----------------------------------------------------------