Context patterns

- can be used to make defaults, constraints, and content descriptions context dependent.

(The context of a node is its sequence of ancestors.)

A context expression is a sequence of context terms:

 contextexp  ->  contextterm*

It matches a context if the context can be decomposed into consecutive fragments, each matched by a context term.

A context term either matches an arbitrary fragment or a single element:

 contextterm  -> <SomeElements/> | 
                 <Element (IDRef="element ID" | Name="element name")^?>
                   attributedescr*
                 </Element>

(Context expressions always implicitly start with a <SomeElements/>.)

DSD context patterns are reminiscent of CSS selectors.

Examples:

XHTML input elements are only allowed inside a form element.
In BookML, the title element is optional inside a book element with an isbn attribute.

XHTML anchors are not allowed to be nested:

    <Constraint>
      <Not>
        <Context>
          <Element Name="a"/><SomeElements/><Element Name="a"/>
        </Context>
      </Not>
    </Constraint>

(This is placed as a constraint to a.)