Theorem: Matroid.freeOn_erk_eq (Matroid/Rank.lean)

State before:
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
Tactic:
rw [erk_eq_er_ground, freeOn_ground, (freeOn_indep_iff.2 rfl.subset).er]
State after:
no goals