exact ⟨e, he.1, h.eq_of_subset_indep (hC.diff_singleton_indep he.1)
  (subset_diff_singleton h.subset he.2) diff_subset⟩
case refine'_1.intro
α : Type u_1
M : Matroid α
C C' I X K C₁ C₂ R : Set α
e✝ f x y : α
hC : M.Circuit C
h : M.Basis I C
e : α
he : e ∈ C ∧ e ∉ I
⊢ ∃ e ∈ C, I = C \ {e}
no goals