rw [delete_dual_eq_dual_contract, contract_dual_eq_dual_delete]
case intro.intro.intro.intro.intro
α : Type u_1
M M' : Matroid α
e f : α
I J R B X Y Z K : Set α
M₀ M₁ M₂ : Matroid α
C D : Set α
⊢ (M ／ C ＼ D)✶ ≤m M✶
case intro.intro.intro.intro.intro
α : Type u_1
M M' : Matroid α
e f : α
I J R B X Y Z K : Set α
M₀ M₁ M₂ : Matroid α
C D : Set α
⊢ M✶ ＼ C ／ D ≤m M✶