convert h.modularPair_compl_biUnion {i₀}
ι : Type u_1
α : Type u_2
M : Matroid α
B I J X X' Y Y' F : Set α
Xs Ys Js Is : ι → Set α
i j : ι
e f : α
h : M.ModularFamily Xs
i₀ : ι
⊢ M.ModularPair (Xs i₀) (⋃ i ∈ {i₀}ᶜ, Xs i)
case h.e'_3
ι : Type u_1
α : Type u_2
M : Matroid α
B I J X X' Y Y' F : Set α
Xs Ys Js Is : ι → Set α
i j : ι
e f : α
h : M.ModularFamily Xs
i₀ : ι
⊢ Xs i₀ = ⋃ i ∈ {i₀}, Xs i