-- Mathlib/AlgebraicGeometry/EllipticCurve/Group.lean
lemma XYIdeal_neg_mul {x y : F} (h : W.Nonsingular x y) :
    XYIdeal W x (C <| W.negY x y) * XYIdeal W x (C y) = XIdeal W x

Proof state:
case h.e'_2.h.inr
F : Type u
inst✝ : Field F
W : Affine F
x y : F
h : W.Nonsingular x y
Y_rw :
  (Y - C (C y)) * (Y - C (C (W.negY x y))) -
      C (X - C x) * (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y) =
    W.polynomial * 1
hy : 2 * y + W.a₁ * x + W.a₃ ≠ 0
⊢ ∃ a a_1 a_2 a_3,
    a_3 * (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y) =
      1 + a * C (X - C x) + a_1 * (Y - C (C y)) + a_2 * (Y - C (C (W.negY x y)))

Tactic:
let W_Y := 2 * y + W.a₁ * x + W.a₃

Result:
case h.e'_2.h.inr
F : Type u
inst✝ : Field F
W : Affine F
x y : F
h : W.Nonsingular x y
Y_rw :
  (Y - C (C y)) * (Y - C (C (W.negY x y))) -
      C (X - C x) * (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y) =
    W.polynomial * 1
hy : 2 * y + W.a₁ * x + W.a₃ ≠ 0
W_Y : F := 2 * y + W.a₁ * x + W.a₃
⊢ ∃ a a_1 a_2 a_3,
    a_3 * (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y) =
      1 + a * C (X - C x) + a_1 * (Y - C (C y)) + a_2 * (Y - C (C (W.negY x y)))

Proof state:
case h.e'_2
F : Type u
inst✝ : Field F
W : Affine F
x y : F
h : W.Nonsingular x y
Y_rw :
  (Y - C (C y)) * (Y - C (C (W.negY x y))) -
      C (X - C x) * (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y) =
    W.polynomial * 1
⊢ Ideal.map (mk W)
      (span
        {C (X - C x), Y - C (C y), Y - C (C (W.negY x y)),
          C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y}) =
    Ideal.map (mk W) ⊤

Tactic:
apply congr_arg

Result:
case h.e'_2.h
F : Type u
inst✝ : Field F
W : Affine F
x y : F
h : W.Nonsingular x y
Y_rw :
  (Y - C (C y)) * (Y - C (C (W.negY x y))) -
      C (X - C x) * (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y) =
    W.polynomial * 1
⊢ span
      {C (X - C x), Y - C (C y), Y - C (C (W.negY x y)),
        C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y} =
    ⊤

Proof state:
case h.e'_2.h
F : Type u
inst✝ : Field F
W : Affine F
x y : F
h : W.Nonsingular x y
Y_rw :
  (Y - C (C y)) * (Y - C (C (W.negY x y))) -
      C (X - C x) * (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y) =
    W.polynomial * 1
⊢ span
      {C (X - C x), Y - C (C y), Y - C (C (W.negY x y)),
        C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y} =
    ⊤

Tactic:
simp_rw [eq_top_iff_one, mem_span_insert', mem_span_singleton']

Result:
case h.e'_2.h
F : Type u
inst✝ : Field F
W : Affine F
x y : F
h : W.Nonsingular x y
Y_rw :
  (Y - C (C y)) * (Y - C (C (W.negY x y))) -
      C (X - C x) * (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y) =
    W.polynomial * 1
⊢ ∃ a a_1 a_2 a_3,
    a_3 * (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y) =
      1 + a * C (X - C x) + a_1 * (Y - C (C y)) + a_2 * (Y - C (C (W.negY x y)))

Proof state:
case h.e'_2.h.inl
F : Type u
inst✝ : Field F
W : Affine F
x y : F
h : W.Nonsingular x y
Y_rw :
  (Y - C (C y)) * (Y - C (C (W.negY x y))) -
      C (X - C x) * (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y) =
    W.polynomial * 1
hx : W.a₁ * y - (3 * x ^ 2 + 2 * W.a₂ * x + W.a₄) ≠ 0
W_X : F := W.a₁ * y - (3 * x ^ 2 + 2 * W.a₂ * x + W.a₄)
⊢ C (C (W_X⁻¹ * -1)) * (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y) =
    1 + C (C W_X⁻¹ * -(X + C (2 * x + W.a₂))) * C (X - C x) + C (C (W_X⁻¹ * W.a₁)) * (Y - C (C y)) +
      0 * (Y - C (C (W.negY x y)))

Tactic:
rw [← mul_right_inj' <| C_ne_zero.mpr <| C_ne_zero.mpr hx]

Result:
case h.e'_2.h.inl
F : Type u
inst✝ : Field F
W : Affine F
x y : F
h : W.Nonsingular x y
Y_rw :
  (Y - C (C y)) * (Y - C (C (W.negY x y))) -
      C (X - C x) * (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y) =
    W.polynomial * 1
hx : W.a₁ * y - (3 * x ^ 2 + 2 * W.a₂ * x + W.a₄) ≠ 0
W_X : F := W.a₁ * y - (3 * x ^ 2 + 2 * W.a₂ * x + W.a₄)
⊢ C (C (W.a₁ * y - (3 * x ^ 2 + 2 * W.a₂ * x + W.a₄))) *
      (C (C (W_X⁻¹ * -1)) * (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y)) =
    C (C (W.a₁ * y - (3 * x ^ 2 + 2 * W.a₂ * x + W.a₄))) *
      (1 + C (C W_X⁻¹ * -(X + C (2 * x + W.a₂))) * C (X - C x) + C (C (W_X⁻¹ * W.a₁)) * (Y - C (C y)) +
        0 * (Y - C (C (W.negY x y))))

Proof state:
case h.e'_2.h.inl
F : Type u
inst✝ : Field F
W : Affine F
x y : F
h : W.Nonsingular x y
Y_rw :
  (Y - C (C y)) * (Y - C (C (W.negY x y))) -
      C (X - C x) * (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y) =
    W.polynomial * 1
hx : W.a₁ * y - (3 * x ^ 2 + 2 * W.a₂ * x + W.a₄) ≠ 0
⊢ ∃ a a_1 a_2 a_3,
    a_3 * (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y) =
      1 + a * C (X - C x) + a_1 * (Y - C (C y)) + a_2 * (Y - C (C (W.negY x y)))

Tactic:
let W_X := W.a₁ * y - (3 * x ^ 2 + 2 * W.a₂ * x + W.a₄)

Result:
case h.e'_2.h.inl
F : Type u
inst✝ : Field F
W : Affine F
x y : F
h : W.Nonsingular x y
Y_rw :
  (Y - C (C y)) * (Y - C (C (W.negY x y))) -
      C (X - C x) * (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y) =
    W.polynomial * 1
hx : W.a₁ * y - (3 * x ^ 2 + 2 * W.a₂ * x + W.a₄) ≠ 0
W_X : F := W.a₁ * y - (3 * x ^ 2 + 2 * W.a₂ * x + W.a₄)
⊢ ∃ a a_1 a_2 a_3,
    a_3 * (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y) =
      1 + a * C (X - C x) + a_1 * (Y - C (C y)) + a_2 * (Y - C (C (W.negY x y)))

Proof state:
F : Type u
inst✝ : Field F
W : Affine F
x y : F
h : W.Nonsingular x y
Y_rw :
  (Y - C (C y)) * (Y - C (C (W.negY x y))) -
      C (X - C x) * (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y) =
    W.polynomial * 1
⊢ span {(mk W) (C (X - C x))} *
      span
        {(mk W) (C (X - C x)), (mk W) (Y - C (C y)), (mk W) (Y - C (C (W.negY x y))),
          (AdjoinRoot.mk W.polynomial) (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y)} =
    XIdeal W x

Tactic:
convert mul_top (_ : Ideal W.CoordinateRing) using 2

Result:
case h.e'_2.h.e'_6
F : Type u
inst✝ : Field F
W : Affine F
x y : F
h : W.Nonsingular x y
Y_rw :
  (Y - C (C y)) * (Y - C (C (W.negY x y))) -
      C (X - C x) * (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y) =
    W.polynomial * 1
⊢ span
      {(mk W) (C (X - C x)), (mk W) (Y - C (C y)), (mk W) (Y - C (C (W.negY x y))),
        (AdjoinRoot.mk W.polynomial) (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y)} =
    ⊤

Proof state:
case h.e'_2.h.e'_6
F : Type u
inst✝ : Field F
W : Affine F
x y : F
h : W.Nonsingular x y
Y_rw :
  (Y - C (C y)) * (Y - C (C (W.negY x y))) -
      C (X - C x) * (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y) =
    W.polynomial * 1
⊢ Ideal.map (mk W)
      (span
        {C (X - C x), Y - C (C y), Y - C (C (W.negY x y)),
          C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y}) =
    ⊤

Tactic:
convert map_top (R := F[X][Y]) (mk W) using 1

Result:
case h.e'_2
F : Type u
inst✝ : Field F
W : Affine F
x y : F
h : W.Nonsingular x y
Y_rw :
  (Y - C (C y)) * (Y - C (C (W.negY x y))) -
      C (X - C x) * (C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y) =
    W.polynomial * 1
⊢ Ideal.map (mk W)
      (span
        {C (X - C x), Y - C (C y), Y - C (C (W.negY x y)),
          C (X ^ 2 + C (x + W.a₂) * X + C (x ^ 2 + W.a₂ * x + W.a₄)) - C (C W.a₁) * Y}) =
    Ideal.map (mk W) ⊤