The XML Path Language (XPath) syntax supports Boolean functions that return strings or numbers, and can be used with comparison operators in filter patterns.
| boolean | Converts the argument to a Boolean. |
| false | Returns false. |
| lang | Returns true if the xml:lang attribute of the context node is the same as the argument string. |
| not | Returns true if the argument is false, otherwise, false. |
| true | Returns true. |