Add Lean statement for 2023 A6

lean4/src/putnam_2023_a6.lean

@@ -0,0 +1,20 @@
+import Mathlib
+
+open Finset
+
+abbrev putnam_2023_a6_solution : Set ℕ := sorry
+-- {n : ℕ | 0 < n}
+
+theorem putnam_2023_a6
+    (IsValidGame : List ℕ → Prop)
+    (IsValidGame_def : ∀ g, IsValidGame g ↔ g.Nodup ∧ (∀ i ∈ g, i ∈ Icc 1 g.length))
+    (parityOf : List ℕ → ZMod 2)
+    (parityOf_def : ∀ g, parityOf g = ((range g.length).filter fun i ↦ g[i]! = i + 1).card)
+    (ConformsToStrategy : List ℕ → (List ℕ → ℕ) → Prop)
+    (ConformsToStrategy_def : ∀ g s, ConformsToStrategy g s ↔
+      ∀ (i) (h : i < g.length), Odd i → g[i] = s (g.take i))
+    (IsWinningFor : ℕ → (List ℕ → ℕ) → Prop)
+    (IsWinningFor_def : ∀ n s, IsWinningFor n s ↔
+      ∃ p, ∀ g, g.length = n → IsValidGame g → ConformsToStrategy g s → parityOf g = p) :
+    {n : ℕ | 0 < n ∧ ∃ s, IsWinningFor n s} = putnam_2023_a6_solution :=
+  sorry