idref

               The @idref attribute is used to specify references to internal
               elements within the same document. The @idref is specified as
               the value of the @id attribute of the element being referenced.
               As in HTML, there is an equivalence between an @href specified
               as href="#{id}" and idref="{id}". However, the @idref attribute
               is preferred for internal references.
               
               If the @idref points to a <ref> element, then the referencing
               element builds on top of that reference, acquiring its attributes
               as default values (which may be overridden by local values). This
               is a recursive structure; an element may, through the @idref 
               attribute, point to an <ref> element which itself builds on 
               another <ref> element, and so on. This is to support the complex
               referencing sometimes found in legislation.