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Boolean expressions

Boolean logic for expressing properties of elements: combined with and, or, not, impl, etc.

Example: 

<or>
  <and>
    <attribute name="a"><stringtype ref="b"/></attribute>
    <ancestor><element name="p:c"><attribute name="d"/></element></ancestor>
  </and>
  <contents>
    <sequence><element name="p:e"/><element name="p:f"/></sequence>
  </contents>
</or>

means: "either the current element has an a attribute with a b value and also a c ancestor element with a d attribute, or - if only looking at e and f elements - its contents consist of one e element followed by one f element (where the elements use the namespace selected by the p prefix)."

Boolean expressions are used both as conditions in conditional constraints (if) and as requirements (in require).

As with the other syntactic categories, boolean expressions can be defined for modularity.

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