have {c x y : Int} (hc : c > 0) : (c * (x - y) < 0) = (x - y < 0) := by
  rw (config := { occs := .pos [1] }) [← Int.mul_zero c, Int.mul_lt_mul_left hc]
a b c d a₁ a₂ b₁ b₂ c₁ c₂ : Int
hc₁ : c₁ > 0
hc₂ : c₂ > 0
h : c₁ * (a₁ - a₂) = c₂ * (b₁ - b₂)
⊢ (a₁ < a₂) = (b₁ < b₂)
a b c d a₁ a₂ b₁ b₂ c₁ c₂ : Int
hc₁ : c₁ > 0
hc₂ : c₂ > 0
h : c₁ * (a₁ - a₂) = c₂ * (b₁ - b₂)
this : ∀ {c x y : Int}, c > 0 → (c * (x - y) < 0) = (x - y < 0)
⊢ (a₁ < a₂) = (b₁ < b₂)