content: personas :: [(String, String)] -> [String]
  personas n = quitarRepetidos (listaAux n)
  
  quitarRepetidos :: [String] -> [String]
  quitarRepetidos [] = []
  quitarRepetidos (x : xs)
    | pertenece x xs = quitarRepetidos xs
    | otherwise = x : quitarRepetidos xs
  
  listaAux :: [(String, String)] -> [String]
  listaAux [] = []
  listaAux ((x, y) : xs) = x : y : listaAux xs
  
  pertenece :: String -> [String] -> Bool
  pertenece x [] = False
  pertenece x (y : ys)
    | x /= y = pertenece x ys
    | otherwise = True
  
  {-
   prueba 1:
  
  Dado: [(Juan, Perez), (Maria, Lopez), (Juan, Perez)]
   es ["Juan", "Perez", "Maria", "Lopez"].
  
  Caso de prueba 2:
  
  Dado: [(Ana, Garcia), (Pedro, Martinez), (Maria, Rodriguez)]
   es ["Ana", "Garcia", "Pedro", "Martinez", "Maria", "Rodriguez"].
  
  Caso de prueba 3:
  
  Dado: [(Juan, Perez), (Juan, Perez), (Juan, Perez)]
  o es ["Juan", "Perez"].
  
  -}