trueif the status of the element specified by element-expression is as specified, and
isntreverses the result.
status (of last subelement)? (of element-expression)? (is | isnt) (proper | inclusion)
This operator determines why an element was allowed in a document.
If the status of the specified element is:
proper, the element indicated by element-expression is a proper element, allowed by the document's content model.
inclusion, the element has been permitted by an inclusion exception.
of last subelement phrase is included, then the test is performed on the last sub-element
of the specified element. Otherwise, the test is performed on the specified element itself.
If the specified element does not exist, the test fails. This is not an error.