simp_rw [(hIs _).er_eq_encard]
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✝ : Fintype ι
h : M.SkewFamily Xs
Is : ι → Set α
hIs : ∀ (i : ι), M.Basis (Is i) (Xs i)
hdj : univ.Pairwise (Disjoint on fun i => Is i)
⊢ ∑ i : ι, M.er (Xs i) = ∑ i : ι, (Is i).encard
no goals