type Name = String
type Table = [ (Name, Name) ]

fathers :: Table
fathers = [
  ("a", "d"),
  ("b", "r")
  ]

head' :: [a] -> a
head' (x:xs) = x


helper :: Integer -> [a] -> Integer
helper acc (x:xs) = helper (acc + 1) xs
helper acc ([]) = acc

len' :: [a] -> Integer
len' xs = helper 0 xs


inv :: [a] -> [a] -> [a]
inv acc (x:xs) = inv (x : acc) xs
inv acc [] = acc

rev :: [a] -> [a]
rev xs = inv [] xs

-- 1 : 2 : 3 : []
-- 3 : 2 : 1 : []


getF :: Name -> Maybe Name
getF n = lookup n fathers