int
MyClass::countItemsIn (...)
  CODE:
    if (SV_IS_MYLIST(ST(0))) {
      THIS->countItemsIn (
        CAST_SV_TO_MYLIST(ST(0))
      );
    } elsif (SV_IS_MYHASH(ST(0)) {
      THIS->countItemsIn (
        CAST_SV_TO_MYHASH(ST(0))
      );
    };
  OUTPUT:
   RETVAL