rw [erk_eq_er_ground, delete_ground, delete_er_eq', diff_diff, union_self, ← er_cl_eq, hX,
  erk_eq_er_ground]
α : Type u_1
M N : Matroid α
I J C D X Y Z : Set α
e f : α
hX : M.cl (M.E \ X) = M.E
⊢ (M ＼ X).erk = M.erk
no goals