∀ {x₁ x₂ y₁ y₂ : SetTheory.PGame},
  (∀ (i : x₂.LeftMoves), Surreal.Multiplication.IH3 x₁ (x₂.moveLeft i) x₂ y₁ y₂) →
    (∀ (i : (-x₁).LeftMoves), Surreal.Multiplication.IH3 (-x₂) ((-x₁).moveLeft i) (-x₁) y₁ y₂) →
      x₁ < x₂ → Surreal.Multiplication.P3 x₁ x₂ y₁ y₂