simp_rw [rFin.skewFamily_iff_sum_er_eq_er_iUnion (fun i ↦ M.to_rFin (Xs i)) hXs,
  ← M.coe_r_eq, ← Nat.cast_sum, Nat.cast_inj]
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
inst✝¹ : Fintype ι
inst✝ : M.FiniteRk
hXs : ∀ (i : ι), Xs i ⊆ M.E
⊢ M.SkewFamily Xs ↔ ∑ i : ι, M.r (Xs i) = M.r (⋃ i, Xs i)
no goals