@calendar indicates the system or calendar to which the date represented by the content of this element belongs, but this element has no textual content. The element should not be categorized in detail with @subtype unless also categorized in general with @type @targetLang can only be used if @target is specified. The element indicated by @spanTo must follow the current element @spanTo on must point to the identifier of some element in this document You must provide either @when or @to/@from, or @notAfter/@notBefore. Only one of the attributes @target and @cRef may be supplied on . Only one of the attributes @target' and @cRef' may be supplied on If the @target attribute on is used, the relatedItem element must be empty A relatedItem element should have either a 'target' attribute or a child element to indicate the related bibliographic item An lg element must contain at least one child l, lg or gap element. The element may appear once only The element may appear once only The element may appear once only You must supply either a locator of some type or a name for Only one of the attributes @active and @mutual may be supplied the attribute 'passive' may be supplied only if the attribute 'active' is supplied The spanTo= attribute of is required. The @spanTo attribute of is required. The spanTo= attribute of is required. must have at least one child add and at least one child del