∀ {F : Type u} [inst : Field F] {W : WeierstrassCurve.Affine F} [inst_1 : DecidableEq F] {x₁ x₂ y₁ y₂ : F},
  x₁ = x₂ →
    y₁ ≠ W.negY x₂ y₂ →
      W.slope x₁ x₂ y₁ y₂ = -Polynomial.evalEval x₁ y₁ W.polynomialX / Polynomial.evalEval x₁ y₁ W.polynomialY