sources>
                <xbl:implementation>
                    <xf:model id="date-model">
                        <xf:instance id="date-instance">
                            <date>
                                <year></year>
                                <month></month>
                                <day></day>
                                <is-valid></is-valid>
                                <is-readonly></is-readonly>
                            </date>
                        </xf:instance>
