Solution2.html

parent :: Sheep -> Maybe Sheep
parent s = father s `mplus` mother s

grandparent :: Sheep -> Maybe Sheep
grandparent s = paternalGrandfather s `mplus` 
                paternalGrandmother s `mplus` 
                maternalGrandfather s `mplus` 
                maternalGrandmother s

Alternative grandparent:

grandparent :: Sheep -> Maybe Sheep
grandparent s = parent s >>= parent