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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
This solution will not work. If the sheep has a father and only a maternal grandparent, this function will return Nothing:
grandparent :: Sheep -> Maybe Sheep
grandparent s = parent s >>= parent