// effective model.
        //
        // As of 3.4, the model version is mandatory even in raw models. The XML element still is optional in the
        // XML schema and this will not change anytime soon. We do not want to build effective models based on
        // models without a version starting with 3.4.
        validateStringNotEmpty("modelVersion", problems, Severity.ERROR, Version.V20, m.getModelVersion(), m);