Theorem: Matroid.er_eq_er_diff_er_le_zero (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
X : Set α
hY : M.er Y ≤ 0
⊢ M.er (X \ Y) = M.er X
Tactic:
rw [← er_eq_er_union_er_le_zero (X \ Y) hY, diff_union_self, er_eq_er_union_er_le_zero _ hY]
State after:
no goals