rw [hI, insert_diff_singleton, insert_eq_of_mem he]
case refine'_1
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
x✝ : ∃ e ∈ C, I = C \ {e}
e : α
he : e ∈ C
hI : I = C \ {e}
⊢ C = insert e I
no goals