∀ {x y : ℝ},
  (((∀ (k : ℤ), x ≠ (2 * ↑k + 1) * Real.pi / 2) ∧ ∀ (l : ℤ), y ≠ (2 * ↑l + 1) * Real.pi / 2) ∨
      (∃ k, x = (2 * ↑k + 1) * Real.pi / 2) ∧ ∃ l, y = (2 * ↑l + 1) * Real.pi / 2) →
    Real.tan (x - y) = (Real.tan x - Real.tan y) / (1 + Real.tan x * Real.tan y)