User-implemented mergesort that exercises features added across the
last few iterations:
let rec split lst = match lst with
| x :: y :: rest ->
let (a, b) = split rest in (* iter 98 let-tuple destruct *)
(x :: a, y :: b)
| ...
let rec merge xs ys = match xs with
| x :: xs' ->
match ys with (* nested match-in-match *)
| y :: ys' -> ...
...
List.fold_left (+) 0 (sort [...]) (* iter 89 (op) section *)
Sum of [3;1;4;1;5;9;2;6;5;3;5] = 44 regardless of order, so the
result is also a smoke test of the implementation correctness — if
merge_sort drops or duplicates an element the sum diverges. 26
baseline programs total.