The history of luaTEX 2006–2009 / v 0.50 - Pragma ADE

These commands tell MkIV that elements with a namespace specication that contains
cals will be remapped to the internal namespace cals and the setup associates a function
with this internal namespace.
By now it will be clear that from the perspective of the user hardly any Lua is visible. Sure,
he or she can deduce that deep down some magic takes place, especially when you run
into more complex expressions like this (the @ denotes an attribute):
\xmlsetsetup
{main} {item[@type='mpctext' or @type='mrtext']}
{questions:multiple:text}
Such expressions resemble xpath, but can go much further than that, just by adding more
functions to the library.
b[position() > 2 and position() < 5 and text() == 'ok']
b[position() > 2 and position() < 5 and text() == upper('ok')]
b[@n=='03' or @n=='08']
b[number(@n)>2 and number(@n)