Class attribute syntax

The class attribute has a particular syntax that must be followed for it to be processed correctly.

Every element must have a class attribute. The class attribute starts with a "-" if it is declared in a structural module, or a "+" if it is declared in a domain module. After the starting token are one or more blank-delimited values, ending with a blank. Each value has two parts: the first part identifies a specialization module, for example a topic type or domain name, and the second part (after a / ) identifies an element type. Structural names are taken from the root element for the topic type or map type. Domain names are defined in the domain module.

Typically, the class attribute value should be declared as a default attribute value in the DTD or schema rather than directly in the document instance. The class attribute should not be modified by the author.

Figure 1. Example structural type element with class attribute

<appstep class="- topic/li task/step bctask/appstep ">A specialized step</appstep>

Figure 2. Example domain element with class attribute

<wintitle class="+ topic/keyword ui-d/wintitle ">A specialized keyword</wintitle>

When the class attribute is declared in the DTD or schema, it must be declared with a default value. In order to support generalization round-tripping (generalizing specialized content into a generic form and then returning it to the specialized form) the default value must not be fixed. This allows the generalization process to overwrite the default values in a general document type with specialized values taken from the document being generalized.

When a specialized type declares new elements, it must provide a class attribute for the new element. The class attribute must include a mapping for every structural type or domain in the specialized type's ancestry, even those in which no element renaming occurred. The mapping should start with the value for the base type (for example topic or map), and finish with the current element type.

Figure 3. Example attribute with intermediate value

<windowname class="- topic/kwd task/kwd guitask/windowname ">

Intermediate values are necessary so that generalizing and specializing transforms can map values simply and accurately. For example, if task/kwd was missing as a value, and a user decided to generalize this guitask up to a task topic, then the transform would have to guess whether to map to kwd (appropriate if task is more general than guitask, which it is) or leave as windowname (appropriate if task were more specialized, which it isn't). By always providing mappings for more general values, we can then apply the simple rule that missing mappings must by default be to more specialized values than the one we are generalizing to, which means the last value in the list is appropriate. For example, when specializing to <task>, if a <p> element has no target value for <task>, we can safely assume that <p> does not specialize from <task> and should not be generalized.

While this example is trivial, more complicated hierarchies (say, five levels deep, with renaming occurring at two and four only) make explicit intermediate values essential.

A specialized type does not need to change the class attribute for elements that it does not specialize, but simply reuses by reference from more generic levels. For example, since task, bctask, and guitask use the p element without specializing it, they don't need to declare mappings for it.

A specialized type only declares class attributes for the elements that it uniquely declares. It does not need to declare class attributes for elements that it reuses or inherits.