open element element-qualifier* (is | isnt) (element-name | element-name-list)

An element test that succeeds if a specified element is opened in the set of opened elements. It is equivalent to, but more efficient than, ancestor is combined with the element is test.

Replacing is with isnt will reverse the results.

If no element qualifiers are present, the test always applies to the current element.

The test allows the programmer to specify a parenthesized list of element-names separated by or or | instead of by a single element name. The test succeeds any of the names in this element-name-list are in the set of opened elements.