ocaml: phase 5.1 powerset_target.ml baseline (subsets of {1..10} summing to 15 = 20)

Recursive powerset construction by doubling:

  let rec gen xs = match xs with
    | [] -> [[]]
    | h :: rest ->
      let sub = gen rest in
      sub @ List.map (fun s -> h :: s) sub

Enumerates all 2^10 = 1024 subsets, filters by sum:

  count = |{ S ⊆ {1..10} | Σ S = 15 }| = 20

Examples: {1,2,3,4,5}, {2,3,4,6}, {1,4,10}, {7,8}, {6,9}, ...

Tests recursive subset construction via List.map + closures,
pattern matching with h :: rest, List.fold_left (+) 0 sum reduce,
exhaustive O(2^n * n) traversal.

172 baseline programs total.

plans/ocaml-on-sx.md

@@ -407,6 +407,15 @@ _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.
+- 2026-05-11 Phase 5.1 — powerset_target.ml baseline (count subsets
+  of {1..10} summing to 15 = 20). Generates the full 2^10 = 1024
+  powerset by recursive doubling: `gen (h :: rest) = gen rest @
+  map (cons h) (gen rest)`, then filters by sum. The 20 valid
+  subsets include {1,2,3,4,5}, {2,3,4,6}, {1,4,10}, {7,8}, {6,9},
+  etc. Tests recursive subset construction via List.map + closures,
+  pattern matching with `h :: rest`, `List.fold_left (+) 0`
+  one-line summary, exhaustive O(2^n · n) traversal. 172 baseline
+  programs total.
 - 2026-05-11 Phase 5.1 — parser: top-level tuple patterns
   (`match e1, e2 with | p1, p2 -> …`). `parse-pattern` now collects
   comma-separated patterns into a `:ptuple`, mirroring the scrutinee