rwa [cl_insert_eq_of_mem_cl hf, diff_self, iff_false_intro (not_mem_empty _)] at he
α : Type u_1
ι : Type u_2
M : Matroid α
F I J X Y B C R : Set α
e f x y : α
he : e ∈ M.cl (insert f X) \ M.cl X
hf : f ∈ M.cl X
⊢ False
no goals