rw [← union_singleton, hIX.er_eq_er_union, 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
hIX : M.Basis I X
e : α
⊢ M.er (insert e I) = M.er (insert e X)
no goals