[Haskell-beginners] Why is this not a "category"?

on https://en.wikibooks.org/w/index.php?title=Haskell/Category_theory&stable=0#Hask.2C_the_Haskell_category the second exercise in the box (see illustration there) asks "(Harder.) If we add another morphism to the above example, it fails to be a category. Why? Hint: think about associativity of the composition operation." There are no answers-to-exercises. Can someone explain to me why adding another function with the same type causes the Haskell type system to no longer form the Hask category? (scratching head)