umEven :: Even Integer String -> Integer
sumEven = cataEven (0 `maybe` uncurry (+)) (\(x, y) -> read x + y)

sumEven :: Even Integer String -> Integer
sumEven = cataEven (0 `maybe` uncurry (+)) (\(x, y) -> read x + y)
sumEven = cataEven (0 `maybe` uncurry (+)) (\(x, y) -> read x + y)