pref :: TermList f -> TermList f -> [a] -> Array (Index f a) -> Index f a -> Stack f a -> Stack f a
pref !_ !_ _ !_ !_ _ | False = undefined
pref :: TermList f -> TermList f -> [a] -> Array (Index f a) -> Index f a -> Stack f a -> Stack f a
pref !_ !_ _ !_ !_ _ | False = undefined
pref !_ !_ _ !_ !_ _ | False = undefined