ocaml: phase 5.1 euler2.ml baseline (Project Euler #2, even Fib <= 4M = 4613732)

Sum of even-valued Fibonacci numbers up to 4,000,000:

  let euler2 limit =
    let a = ref 1 in
    let b = ref 2 in
    let sum = ref 0 in
    while !a <= limit do
      if !a mod 2 = 0 then sum := !sum + !a;
      let c = !a + !b in
      a := !b;
      b := c
    done;
    !sum

Sequence: 1, 2, 3, 5, 8, 13, 21, 34, ... Only every third term
(2, 8, 34, 144, ...) is even. Sum below 4M: 4613732.

101 baseline programs total.

lib/ocaml/baseline/euler2.ml

@@ -0,0 +1,15 @@
+let euler2 limit =
+  let a = ref 1 in
+  let b = ref 2 in
+  let sum = ref 0 in
+  while !a <= limit do
+    if !a mod 2 = 0 then sum := !sum + !a;
+    let c = !a + !b in
+    a := !b;
+    b := c
+  done;
+  !sum
+
+;;
+
+euler2 4000000

lib/ocaml/baseline/expected.json

@@ -25,6 +25,7 @@
   "exception_handle.ml": 4,
   "exception_user.ml": 26,
   "euler1.ml": 233168,
+  "euler2.ml": 4613732,
   "expr_eval.ml": 16,
   "expr_simp.ml": 22,
   "factorial.ml": 3628800,

plans/ocaml-on-sx.md

@@ -407,6 +407,11 @@ _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 — euler2.ml baseline (Project Euler #2, sum
+  of even Fibonacci ≤ 4M = 4613732). Iterative two-ref Fibonacci,
+  accumulating only even terms. Sequence: 1, 2, 3, 5, 8, 13, 21,
+  34... only 2, 8, 34, 144, ... contribute. 101 baseline programs
+  total.
 - 2026-05-09 Phase 5.1 — euler1.ml baseline (Project Euler #1, sum
   of multiples of 3 or 5 below 1000 = 233168). Trivial DSL exercise
   but symbolically meaningful: this is the 100th baseline program.