rw [← union_singleton, er_union_cl_left_eq, union_singleton]
α : Type u_1
ι : Type u_2
M✝ N : Matroid α
I B X✝ X' Y Y' Z R : Set α
n : ℕ∞
e✝ f : α
M : Matroid α
e : α
X : Set α
⊢ M.er (insert e (M.cl X)) = M.er (insert e X)
no goals