public virtual class C1 { }
public virtual class C2 extends C1 { }
// ...
public virtual class CN extends CM { }

virtual class C1 {}
virtual class C2 extends C1 {}
virtual class C3 extends C2 {}
virtual class C4 extends C3 {}
virtual class C5 extends C4 {}
virtual class C6 extends C5 {}
virtual class C7 extends C6 {}
virtual class C8 extends C7 {}
virtual class C9 extends C8 {}
virtual class C10 extends C9 {}
virtual class C11 extends C10 {}
virtual class C12 extends C11 {}