Creates a reference to an XQuery function which can later be called from util:call. This allows for higher-order functions to be implemented in XQuery. A higher-order function is a function that takes another function as argument. The first argument represents the name of the function, which should bea valid QName. The second argument is the arity (number of parameters) of the function. If no function can be found that matches the name and arity, an error is thrown. Please note: the arguments to this function have to be literals or need to be resolvable at compile time at least.