rw [erk_eq_er_ground, freeOn_ground, (freeOn_indep_iff.2 rfl.subset).er]
α : Type u_1
ι : Type u_2
M N : Matroid α
I B X X' Y Y' Z R : Set α
n : ℕ∞
e f : α
E✝ E : Set α
⊢ (freeOn E).erk = E.encard
no goals