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Declare a class in the scope of a interface or class is supported. But declare an interface in the scope of a class or a interface is not supported.

So this is code is invalid:

public interface IAppTop : Object {
  public abstract void method ();
  
  public interface IAppDown : Object {
    public abstract void method ();
  }

}
public class App : Object {
  public interface IApp : Object {
    public abstract string val { get; }
  }
  public static void main () {
    var a = new App;
  }
}

This is because there is no way to add an interface in a ObjectTypeSymbol.