rwa [deleteElem, delete_eq_restrict, restrict_eq_self_iff, sdiff_eq_left,
  disjoint_singleton_right]
α : Type u_1
M M' N : Matroid α
e f : α
I J R✝ B X Y Z K D D₁ D₂ R : Set α
he : e ∉ M.E
⊢ M ＼ e = M
no goals