nil = CON(Nil);
zero = CON(I 0);
one = CON(I 1);
two = CON(I 2);
three = CON(I 3);
 
plusInt = FUN(x y ->
   case x of {
      I i -> case y of {
               I j -> case plus# i j of {
                         x -> let { result = CON (I x) } in result }}});
 
foldl = FUN(f acc list ->
   case list of {
      Nil -> acc;
      Cons h t -> let { newAcc = THUNK(f acc h) } in foldl f newAcc t
   });
 
# lazy sum with a well-known space leak
sum = FUN(list -> foldl plusInt zero list);
 
list1 = CON(Cons one nil);
list2 = CON(Cons two list1);
list3 = CON(Cons three list2);
 
main = THUNK(sum list3);