Lean4
/-- `Sym2.GameAdd rα x y` means that `x` can be reached from `y` by decreasing either entry with
respect to the relation `rα`.
See `Prod.GameAdd` for the ordered pair analog. -/
def GameAdd (rα : α → α → Prop) : Sym2 α → Sym2 α → Prop :=
Sym2.lift₂
⟨fun a₁ b₁ a₂ b₂ => Prod.GameAdd rα rα (a₁, b₁) (a₂, b₂) ∨ Prod.GameAdd rα rα (b₁, a₁) (a₂, b₂), fun a₁ b₁ a₂ b₂ =>
by
dsimp
rw [Prod.gameAdd_swap_swap_mk _ _ b₁ b₂ a₁ a₂, Prod.gameAdd_swap_swap_mk _ _ a₁ b₂ b₁ a₂]
simp [or_comm]⟩