simp only [mem_insert_iff, heI, or_false] at h
case refine_1.inr
α : Type u_1
M : Matroid α
I J B F₀ F F' X Y : Set α
e f : α
U : M.ModularCut
hX : X ⊆ insert e M.E
hI : U.ExtIndep e I
hIX : I ⊆ X
hss : I \ {e} ⊆ X \ {e}
hX' : X \ {e} ⊆ M.E
h :
  ∀ x ∉ I,
    x ∈ X →
      (M.Indep (insert x I) → e ∈ insert x I) ∧
        (M.Indep (insert x I \ {e}) → e ∈ insert x I → M.cl (insert x I \ {e}) ∈ U)
heI : e ∉ I
⊢ M.cl (I \ {e}) = M.cl (X \ {e}) ∧ (e ∈ X → ¬(e ∈ I ↔ M.cl (X \ {e}) ∈ U)) ∨
    (M.cl (I \ {e}) ⋖[M] M.cl (X \ {e})) ∧ e ∈ I ∧ M.cl (X \ {e}) ∈ U
case refine_1.inr
α : Type u_1
M : Matroid α
I J B F₀ F F' X Y : Set α
e f : α
U : M.ModularCut
hX : X ⊆ insert e M.E
hI : U.ExtIndep e I
hIX : I ⊆ X
hss : I \ {e} ⊆ X \ {e}
hX' : X \ {e} ⊆ M.E
heI : e ∉ I
h : ∀ x ∉ I, x ∈ X → (M.Indep (insert x I) → e = x) ∧ (M.Indep (insert x I \ {e}) → e = x → M.cl (insert x I \ {e}) ∈ U)
⊢ M.cl (I \ {e}) = M.cl (X \ {e}) ∧ (e ∈ X → ¬(e ∈ I ↔ M.cl (X \ {e}) ∈ U)) ∨
    (M.cl (I \ {e}) ⋖[M] M.cl (X \ {e})) ∧ e ∈ I ∧ M.cl (X \ {e}) ∈ U