ocaml: phase 5.1 flatten_tree.ml baseline (parametric ADT flatten, sum 1..7 = 28)

Defines a parametric tree:

  type 'a tree = Leaf of 'a | Node of 'a tree list

  let rec flatten t =
    match t with
    | Leaf x -> [x]
    | Node ts -> List.concat (List.map flatten ts)

Test tree has 3 levels of nesting:

  Node [Leaf 1; Node [Leaf 2; Leaf 3];
        Node [Node [Leaf 4]; Leaf 5; Leaf 6];
        Leaf 7]

flattens to [1;2;3;4;5;6;7] -> sum = 28.

Tests parametric ADT, mutual recursion via map+self, List.concat.

55 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,13 @@ _Newest first._
   binary search tree (`type 'a tree = Leaf | Node of 'a * 'a tree *
   'a tree`) with insert + in-order traversal. Tests parametric ADT,
   recursive match, List.append, List.fold_left.
+- 2026-05-09 Phase 5.1 — flatten_tree.ml baseline (parametric ADT
+  flatten, sum 1..7 = 28). Defines `type 'a tree = Leaf of 'a |
+  Node of 'a tree list` then `flatten` recursively expands using
+  `List.concat (List.map flatten ts)`. Tree has 3 levels of
+  nesting; flattens to [1;2;3;4;5;6;7]. Tests parametric ADT, mutual
+  recursion via map+self, List.concat from runtime. 55 baseline
+  programs total.
 - 2026-05-09 Phase 5.1 — gcd_lcm.ml baseline (Euclidean gcd + lcm,
   12 + 12 + 36 = 60). Two-line baseline: `let rec gcd a b = if b = 0
   then a else gcd b (a mod b)` + `let lcm a b = a * b / gcd a b`.