-- Mathlib/AlgebraicGeometry/EllipticCurve/Group.lean
lemma XYIdeal_mul_XYIdeal {x₁ x₂ y₁ y₂ : F} (h₁ : W.Equation x₁ y₁) (h₂ : W.Equation x₂ y₂)
    (hxy : ¬(x₁ = x₂ ∧ y₁ = W.negY x₂ y₂)) :
    XIdeal W (W.addX x₁ x₂ <| W.slope x₁ x₂ y₁ y₂) * (XYIdeal W x₁ (C y₁) * XYIdeal W x₂ (C y₂)) =
      YIdeal W (linePolynomial x₁ y₁ <| W.slope x₁ x₂ y₁ y₂) *
        XYIdeal W (W.addX x₁ x₂ <| W.slope x₁ x₂ y₁ y₂)
          (C <| W.addY x₁ x₂ y₁ <| W.slope x₁ x₂ y₁ y₂)

Proof state:
case h.e'_2.h.e'_5
F : Type u
inst✝ : Field F
W : Affine F
x₁ x₂ y₁ y₂ : F
h₁ : W.Equation x₁ y₁
h₂ : W.Equation x₂ y₂
hxy : ¬(x₁ = x₂ ∧ y₁ = W.negY x₂ y₂)
sup_rw : ∀ (a b c d : Ideal W.CoordinateRing), a ⊔ (b ⊔ (c ⊔ d)) = a ⊔ d ⊔ b ⊔ c
⊢ ∃ x,
    (∃ a a_1 a_2, a_2 * (Y - W.negPolynomial) = x + a * C (X - C x₁) + a_1 * C (X - C x₂)) ∧
      (AdjoinRoot.mk W.polynomial) x = 1

Tactic:
by_cases hx : x₁ = x₂

Result:
case pos
F : Type u
inst✝ : Field F
W : Affine F
x₁ x₂ y₁ y₂ : F
h₁ : W.Equation x₁ y₁
h₂ : W.Equation x₂ y₂
hxy : ¬(x₁ = x₂ ∧ y₁ = W.negY x₂ y₂)
sup_rw : ∀ (a b c d : Ideal W.CoordinateRing), a ⊔ (b ⊔ (c ⊔ d)) = a ⊔ d ⊔ b ⊔ c
hx : x₁ = x₂
⊢ ∃ x,
    (∃ a a_1 a_2, a_2 * (Y - W.negPolynomial) = x + a * C (X - C x₁) + a_1 * C (X - C x₂)) ∧
      (AdjoinRoot.mk W.polynomial) x = 1

case neg
F : Type u
inst✝ : Field F
W : Affine F
x₁ x₂ y₁ y₂ : F
h₁ : W.Equation x₁ y₁
h₂ : W.Equation x₂ y₂
hxy : ¬(x₁ = x₂ ∧ y₁ = W.negY x₂ y₂)
sup_rw : ∀ (a b c d : Ideal W.CoordinateRing), a ⊔ (b ⊔ (c ⊔ d)) = a ⊔ d ⊔ b ⊔ c
hx : ¬x₁ = x₂
⊢ ∃ x,
    (∃ a a_1 a_2, a_2 * (Y - W.negPolynomial) = x + a * C (X - C x₁) + a_1 * C (X - C x₂)) ∧
      (AdjoinRoot.mk W.polynomial) x = 1

Proof state:
F : Type u
inst✝ : Field F
W : Affine F
x₁ x₂ y₁ y₂ : F
h₁ : W.Equation x₁ y₁
h₂ : W.Equation x₂ y₂
hxy : ¬(x₁ = x₂ ∧ y₁ = W.negY x₂ y₂)
sup_rw : ∀ (a b c d : Ideal W.CoordinateRing), a ⊔ (b ⊔ (c ⊔ d)) = a ⊔ d ⊔ b ⊔ c
⊢ span {(mk W) (Y - C (linePolynomial x₁ y₁ (W.slope x₁ x₂ y₁ y₂)))} *
      (span {(mk W) (W.negPolynomial - C (linePolynomial x₁ y₁ (W.slope x₁ x₂ y₁ y₂)))} ⊔
        (span {XClass W x₁} ⊔ (span {XClass W x₂} ⊔ span {(mk W) (Y - W.negPolynomial)})) *
          span {XClass W (W.addX x₁ x₂ (W.slope x₁ x₂ y₁ y₂))}) =
    YIdeal W (linePolynomial x₁ y₁ (W.slope x₁ x₂ y₁ y₂)) *
      (span {(mk W) (W.negPolynomial - C (linePolynomial x₁ y₁ (W.slope x₁ x₂ y₁ y₂)))} ⊔
        span {XClass W (W.addX x₁ x₂ (W.slope x₁ x₂ y₁ y₂))})

Tactic:
apply congr_arg (_ ∘ _)

Result:
F : Type u
inst✝ : Field F
W : Affine F
x₁ x₂ y₁ y₂ : F
h₁ : W.Equation x₁ y₁
h₂ : W.Equation x₂ y₂
hxy : ¬(x₁ = x₂ ∧ y₁ = W.negY x₂ y₂)
sup_rw : ∀ (a b c d : Ideal W.CoordinateRing), a ⊔ (b ⊔ (c ⊔ d)) = a ⊔ d ⊔ b ⊔ c
⊢ (span {XClass W x₁} ⊔ (span {XClass W x₂} ⊔ span {(mk W) (Y - W.negPolynomial)})) *
      span {XClass W (W.addX x₁ x₂ (W.slope x₁ x₂ y₁ y₂))} =
    span {XClass W (W.addX x₁ x₂ (W.slope x₁ x₂ y₁ y₂))}

Proof state:
F : Type u
inst✝ : Field F
W : Affine F
x₁ x₂ y₁ y₂ : F
h₁ : W.Equation x₁ y₁
h₂ : W.Equation x₂ y₂
hxy : ¬(x₁ = x₂ ∧ y₁ = W.negY x₂ y₂)
sup_rw : ∀ (a b c d : Ideal W.CoordinateRing), a ⊔ (b ⊔ (c ⊔ d)) = a ⊔ d ⊔ b ⊔ c
⊢ span {(mk W) (Y - C (linePolynomial x₁ y₁ (W.slope x₁ x₂ y₁ y₂)))} *
      (span {(mk W) (W.negPolynomial - C (linePolynomial x₁ y₁ (W.slope x₁ x₂ y₁ y₂)))} ⊔
            span {(mk W) (Y - C (linePolynomial x₁ y₁ (W.slope x₁ x₂ y₁ y₂)))} *
              span {XClass W (W.addX x₁ x₂ (W.slope x₁ x₂ y₁ y₂))} ⊔
          span {XClass W x₁} * span {XClass W (W.addX x₁ x₂ (W.slope x₁ x₂ y₁ y₂))} ⊔
        span {XClass W x₂} * span {XClass W (W.addX x₁ x₂ (W.slope x₁ x₂ y₁ y₂))}) =
    YIdeal W (linePolynomial x₁ y₁ (W.slope x₁ x₂ y₁ y₂)) *
      (span {(mk W) (W.negPolynomial - C (linePolynomial x₁ y₁ (W.slope x₁ x₂ y₁ y₂)))} ⊔
        span {XClass W (W.addX x₁ x₂ (W.slope x₁ x₂ y₁ y₂))})

Tactic:
rw [span_singleton_mul_span_singleton, ← span_insert,
  ← span_pair_add_mul_right <| -(XClass W <| W.addX x₁ x₂ <| W.slope x₁ x₂ y₁ y₂), mul_neg,
  ← sub_eq_add_neg, ← sub_mul, ← map_sub <| mk W, sub_sub_sub_cancel_right, span_insert,
  ← span_singleton_mul_span_singleton, ← sup_rw, ← Ideal.sup_mul, ← Ideal.sup_mul]

Result:
F : Type u
inst✝ : Field F
W : Affine F
x₁ x₂ y₁ y₂ : F
h₁ : W.Equation x₁ y₁
h₂ : W.Equation x₂ y₂
hxy : ¬(x₁ = x₂ ∧ y₁ = W.negY x₂ y₂)
sup_rw : ∀ (a b c d : Ideal W.CoordinateRing), a ⊔ (b ⊔ (c ⊔ d)) = a ⊔ d ⊔ b ⊔ c
⊢ span {(mk W) (Y - C (linePolynomial x₁ y₁ (W.slope x₁ x₂ y₁ y₂)))} *
      (span {(mk W) (W.negPolynomial - C (linePolynomial x₁ y₁ (W.slope x₁ x₂ y₁ y₂)))} ⊔
        (span {XClass W x₁} ⊔ (span {XClass W x₂} ⊔ span {(mk W) (Y - W.negPolynomial)})) *
          span {XClass W (W.addX x₁ x₂ (W.slope x₁ x₂ y₁ y₂))}) =
    YIdeal W (linePolynomial x₁ y₁ (W.slope x₁ x₂ y₁ y₂)) *
      (span {(mk W) (W.negPolynomial - C (linePolynomial x₁ y₁ (W.slope x₁ x₂ y₁ y₂)))} ⊔
        span {XClass W (W.addX x₁ x₂ (W.slope x₁ x₂ y₁ y₂))})

Proof state:
F : Type u
inst✝ : Field F
W : Affine F
x₁ x₂ y₁ y₂ : F
h₁ : W.Equation x₁ y₁
h₂ : W.Equation x₂ y₂
hxy : ¬(x₁ = x₂ ∧ y₁ = W.negY x₂ y₂)
sup_rw : ∀ (a b c d : Ideal W.CoordinateRing), a ⊔ (b ⊔ (c ⊔ d)) = a ⊔ d ⊔ b ⊔ c
⊢ (span {XClass W x₁} ⊔ (span {XClass W x₂} ⊔ span {(mk W) (Y - W.negPolynomial)})) *
      span {XClass W (W.addX x₁ x₂ (W.slope x₁ x₂ y₁ y₂))} =
    span {XClass W (W.addX x₁ x₂ (W.slope x₁ x₂ y₁ y₂))}

Tactic:
convert top_mul (_ : Ideal W.CoordinateRing)

Result:
case h.e'_2.h.e'_5
F : Type u
inst✝ : Field F
W : Affine F
x₁ x₂ y₁ y₂ : F
h₁ : W.Equation x₁ y₁
h₂ : W.Equation x₂ y₂
hxy : ¬(x₁ = x₂ ∧ y₁ = W.negY x₂ y₂)
sup_rw : ∀ (a b c d : Ideal W.CoordinateRing), a ⊔ (b ⊔ (c ⊔ d)) = a ⊔ d ⊔ b ⊔ c
⊢ span {XClass W x₁} ⊔ (span {XClass W x₂} ⊔ span {(mk W) (Y - W.negPolynomial)}) = ⊤