 a) es la cadena correspondiente al ABB a. Por ejemplo,
--    λ> escribeABB (crea (reverse [5,2,6,4,8,3,9]))
--    " (5 (2 - (4 (3 - -) -)) (6 - (8 - (9 - -))))"
--    λ> escribeABB (foldr inserta vacio (reverse [5,2,4,3,8,6,7,10,9,11]))
--    " (5 (2 - (4 (3 - -) -)) (8 (6 - (7 - -)) (10 (9 - -) (11 - -))))"
escribeABB :: Show a => ABB a -> String
escribeABB Vacio        = " -"
escribeABB (Nod
escribeABB :: Show a => ABB a -> String
escribeABB Vacio        = " -"
escribeABB Vacio        = " -"