Theorem: Matroid.er_le_er_inter_add_er_diff (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 Y : Set α
⊢ M.er X ≤ M.er (X ∩ Y) + M.er (X \ Y)
Tactic:
nth_rw 1 [← inter_union_diff X Y]
State after:
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 Y : Set α
⊢ M.er (X ∩ Y ∪ X \ Y) ≤ M.er (X ∩ Y) + M.er (X \ Y)

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 Y : Set α
⊢ M.er (X ∩ Y ∪ X \ Y) ≤ M.er (X ∩ Y) + M.er (X \ Y)
Tactic:
apply er_union_le_er_add_er
State after:
no goals