Javadoc
This is a special pattern that matches the "anchor node". It is used for the selectors
that arise when evaluating XPath expressions in streaming mode; the anchor
node is the context node for the streamed XPath evaluation.
Given a streamed evaluation of an expression such as ./BOOKS/BOOK/PRICE, the way we evaluate
this is to turn it into a pattern, which is then tested against all descendant nodes.
Conceptually the pattern is $A/BOOKS/BOOK/PRICE, where $A is referred to as the anchor
node. When we evaluate the pattern against (say) a PRICE element, the match will only succeed
if the name of the element is "PRICE" and its ancestors are, in order, a BOOK element, a
BOOKS element, and the anchor node $A.