<xsd:attributeGroup id="attGp.periodic" name="periodic">
    <xsd:attribute name="periodic" type="xsd:boolean" id="att.periodic" default="true">
        <xsd:annotation>
            <xsd:documentation>
                <h:div class="summary" xmlns:h="http://www.w3.org/1999/xhtml">Is the axis periodic.</h:div>
                <h:div class="description" xmlns:h="http://www.w3.org/1999/xhtml">Any or all of the axes may be periodic or aperiodic. An example could be a surface where 2 periodic axes (not necessarily orthogonal) are used to describe the coordinates in the surface, perhaps representing lattice vectors of a 3D crystal or 2D layer. The third vector is orthogonal and represents coordinates normal to the surface. In this case only the direction, not the magnitude of the vector is important.</h:div>
            </xsd:documentation>
        </xsd:annotation>
    </xsd:attribute>
</xsd:attributeGroup>