private (embedded) classes, structs, or enums

[Bernd]

I am seeking a way to define the model such that the C# code generator creates private (or protected) definitions for a certain class, struct or enumeration within another class like so:

public class X {
  private class Y { // or enum, or struct
     ...
  } // end of Y
  ...
} // end of X

Any clues?
TIA,
Bernd