For guile 2.9

Executing

(use-modules (oop goops))
(define-method (< (s1 <string>) (s2 <string>)) (pk 'wow) #f)
(define (f x) (< x x))

(f "a")

results in :

unnamed port>:3:14: In procedure f:

In procedure nan?: Wrong type argument in position 1: "a"