∀ {R : Type u} [inst : Semiring R] [inst_1 : Preorder R] {a b c d : R} [ExistsAddOfLE R] [PosMulMono R] [MulPosMono R]
  [AddRightMono R] [AddRightReflectLE R], c ≤ a → b ≤ d → 0 ≤ a → d ≤ 0 → a * b ≤ c * d