define-xpath-function/lazy (ext name args &body body)

Defines an XPath function, "lazy" style.

The body is evaluated during compilation of XPath expressions each time the function being defined is referenced. It's passed a list of "thunks" corresponding to XPath function arguments and should return a new "thunk". A "thunk" is a function that takes an XPath context as argument and returns value of one of XPath types (string, boolean, number, node set).

Example:

   (define-xpath-function/lazy my-ext my-if (v if-part else-part)
     #'(lambda (ctx)
         (if (boolean-value (funcall v ctx))
             (funcall if-part ctx)
             (funcall else-part ctx))))