Use infix version

VL2/content/VL2_rec_patterns9.tex

@@ -2,7 +2,7 @@ To cut the story short, the abstract solution looks like this:
 \begin{haskellcode}
 fold :: b -> (a -> b -> b) -> [a] -> b
 fold z f []     = z
-fold z f (x:xs) = f x (fold z f xs)
+fold z f (x:xs) = x `f` (fold z f xs)
 \end{haskellcode}
 Whoa! What's going on here?\\
 Let's see...