How to reference the NEW root of the database in the rule expression?

To illustrate my use case, consider a two-way relationship between users and groups. Users can belong to a group, and groups comprise a list of users.

{
  "users": {
    "alovelace": {
      "name": "Ada Lovelace",
      "groupId": "techpioneers",
    },
    "eclarke": {
      "name": "Edmund M. Clarke"
    },
    // ...
  },
  "groups": {
    "techpioneers": {
      "name": "Historical Tech Pioneers",
      "members": {
        "alovelace": true,
        "ghopper": true
      }
    },
    // ...
  }
}

To associate eclarke to a group, the client should update both paths (/users/eclarke/groupId and /groups/techpioneers/members/eclarke) in a single update so that there's no intermidiate invalid state.

To ensure this, I set the security rules for each path to reference each other:

{
  "rules": {
    "users": {
      "$userId": {
        "groupId": {
          ".validate": "newData.parent().parent().parent().child('groups').child(newData.val()).child('members').hasChild($userId)"
        }
      }
    },
    "groups": {
      "$groupId": {
        "members": {
          "$memberId": {
            ".validate": "newData.parent().parent().parent().child('users').child($memberId).child('groupId').val() === $groupId"
          }
        }
      }
    }
  }
}

I cannot use root here becaue I would be looking at old database state.

Is there a better way to do this? (without having to make multiple .parent() calls)