rwa [delete_delete, union_diff_self, union_comm, ← delete_delete, eq_comm,
  delete_eq_self_iff]
case intro.intro
α : Type u_1
M M' : Matroid α
e f : α
I J R✝ B X Y Z K D✝ D₁ D₂ R D : Set α
hD : D ⊆ M.E
h : M ＼ D ≤r M
hX : Disjoint X (M ＼ D).E
⊢ M ＼ D = M ＼ X ＼ (D \ X)
no goals