overriding xpath functions

Hello,
maybe dumb question - what happens when a processor encounters an
expression using the eq operator (and the other related operators)? Since
eq is sort of overloaded and calls different functions according to the
type of the operands (op:numeric-equal, fn:compare etc).
And especially when the operands are empty sequences - like in () eq ()?
The result is an empty sequence*, but what steps lead to it?
Follow-up question - is it possible to override the built-in xpath
functions in XSLT or XQuery?

* is it written somewhere in the specs, that eq returns empty sequence
when some of the operands is an empty sequence? I looked, but did not find
it..

Thanks,
Jakub.