sclass TypeSystem<T> {
  // override if you have supertype (subtype) relations in your type system
  // you can assume neq(a, b)
  bool isActualSupertypeOf(T a, T b) { false; }
  
  bool isSupertypeOf(T a, T b) {
    ret eq(a, b) || isActualSupertypeOf(a, b);
  }
  
  bool operatorCanTakeArguments(GOperator<T> op, T... types) {
    ret matchTwoLists(lambda2 isSupertypeOf,
      reallyLazyMap(op.arguments, a -> a.type()), types);
  }
}