let {
  as =
    {
      x = 123;
      y = 456;
    }
    // {
      z = 789;
    }
    // {
      z = 987;
    };

  A = "a";
  Z = "z";

  body =
    if builtins.hasAttr A as then
      builtins.getAttr A as
    else
      assert builtins.hasAttr Z as;
      builtins.getAttr Z as;
}
