ocaml: phase 5.1 btree.ml baseline (13/13 pass)

Polymorphic binary search tree with insert + in-order traversal.
Exercises parametric ADT (type 'a tree = Leaf | Node of 'a * 'a tree
* 'a tree), recursive match, List.append, List.fold_left.

lib/ocaml/baseline/btree.ml

@@ -0,0 +1,25 @@
+(* Baseline: binary search tree with insert + in-order traversal *)
+type 'a tree =
+  | Leaf
+  | Node of 'a * 'a tree * 'a tree
+;;
+
+let rec insert x t =
+  match t with
+  | Leaf -> Node (x, Leaf, Leaf)
+  | Node (v, l, r) ->
+    if x < v then Node (v, insert x l, r)
+    else if x > v then Node (v, l, insert x r)
+    else t
+;;
+
+let rec inorder t =
+  match t with
+  | Leaf -> []
+  | Node (v, l, r) -> List.append (inorder l) (v :: inorder r)
+;;
+
+let from_list xs = List.fold_left (fun t x -> insert x t) Leaf xs ;;
+
+let t = from_list [5; 3; 8; 1; 4; 7; 9; 2] ;;
+List.fold_left (fun a b -> a + b) 0 (inorder t)

lib/ocaml/baseline/expected.json

@@ -1,4 +1,5 @@
 {
+  "btree.ml": 39,
   "calc.ml": 13,
   "closures.ml": 315,
   "exception_handle.ml": 4,

plans/ocaml-on-sx.md

@@ -399,6 +399,10 @@ _Newest first._
   recognise `!` as the prefix-deref of an application argument, so
   `String.concat "" (List.rev !b)` parses as `(... (deref b))`. Buffer
   uses a ref holding a string list; contents reverses and concats.
+- 2026-05-08 Phase 5.1 — btree.ml baseline (13/13 pass). Polymorphic
+  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.
 - 2026-05-08 Phase 5.1 — fizzbuzz.ml baseline (12/12 pass). Classic
   fizzbuzz using ref-cell accumulator, for-loop, mod, if/elseif chain,
   String.concat, Int.to_string. Verifies output via String.length.