The XPath id() function
XPath 2.0 version: accepts any sequence as the first parameter; each item in the sequence
is taken as an IDREFS value, that is, a space-separated list of ID values.
Also accepts an optional second argument to identify the target document, this
defaults to the context node.
Get the static properties of this expression (other than its type). The result is
bit-signficant. These properties are used for optimizations. In general, if
property bit is set, it is true, but if it is unset, the value is unknown.