int
MyClass::countItemsIn (...)
  CODE:
    if (sv_derived_from(ST(0), 'MyList')) {
      RETVAL=THIS->countItemsIn(
        *(MyList *) SvIV((SV*)SvRV(ST(0)))
      );
    } elsif (sv_derived_from(ST(0), 'MyHash')) {
      RETVAL=THIS->countItemsIn(
        *(MyHash *) SvIV((SV*)SvRV(ST(0)))
      );
    }
  OUTPUT:
    RETVAL