2.1) 1.0 ponto
sum2tips (Tip a) = (Tip a)
sum2tips (Bin (Tip a) (Tip b)) = Tip (a+b)
sum2tips (Bin a b) = Bin (sum2tips a) (sum2tips b)

2.2) 1.0 ponto
sumtree (Tip t) = t
sumtree (Bin t1 t2) = sumtree (sum2tips (Bin t1 t2))

2.3) 0.5 ponto
ziptree (Tip a) (Tip b) = Tip (a,b)
ziptree (Bin t1 t2) (Bin t3 t4) = Bin (ziptree t1 t3) (ziptree t2 t4)

3.1) 1.0 ponto
direto a (No b gs) | a == b = map vals gs
                   | otherwise = concat (map (direto a) gs)

vals (No a gs) = a

3.2) 1.0 ponto
indir a (No b gs) | a == b = concat (map todos gs)
                  | otherwise = concat (map (indir a) gs)

todos (No b gs) = b : concat (map todos gs)

3.3) 1.0 ponto
gtol (No b gs) = map (pair b) gs ++ concat (map gtol gs)
where pair x (No b gs) = (x,b)

4.1) 0.5 ponto
fliptree (Tip a) = Tip a
fliptree (Bin t1 t2) = Bin (fliptree t2) (fliptree t1)

4.2) 1.0 ponto
caso xs = Tip a:
fliptree (fliptree (Tip a)) = fliptree (Tip a) = Tip a

caso xs = Bin t1 t2, hipoteses de inducao:
fliptree (fliptree t1) = t1 e fliptree (fliptree t2) = t2
fliptree (fliptree (Bin t1 t2)) 
= fliptree (Bin (fliptree t2) (fliptree t1))
= Bin (fliptree (fliptree t1)) (fliptree (fliptree t2))
= Bin t1 t2

5) 1.5 ponto
enum arvore = en 1 arvore
where en n (Tip a) = Tip (n,a)
      en n (Bin t1 t2) = Bin (en n t1) (en (n+size t1) t2)

(existem solucoes melhores para esta ultima questao,
atravessando a arvore uma unica vez (i.e sem usar size), mas
a solucao acima e' bem simples)