Firestore 错误函数未找到错误：名称：[存在]

Question

鉴于以下 firestore rules

service cloud.firestore {
  match /databases/{database}/documents {

    function isSuperAdmin() {
      return exists(/databases/$(database)/documents/super_admins/$(request.auth.uid));
    }

    match /projects/{anyProject} {
      match /tasks/{anyTask} {
        allow update: if isSuperAdmin();
      }
    }
  }
}

试图在模拟器中运行一个简单的更新导致以下错误 - Error running simulation — Error: simulator.rules line [20], column [14]. Function not found error: Name: [exists].