Cyclic class inheritance

You cannot have inheritance cycles with class declarations:

class A extends B; endclass
class B extends A; endclass
// this would introduce an infinite loop: A -> B -> A -> B ...

class A#(type T=int) extends T; endclass
class B extends A#(A#(B)); endclass
// this would introduce an infinite loop: B -> A#(A#(B)) -> A#(B) -> B ...