Theorem: Matroid.contract_ground_self (Matroid/Minor/Basic.lean)

State before:
α : Type u_1
M✝ M' N : Matroid α
e f : α
I J R B X Y Z K E : Set α
M : Matroid α
⊢ M ／ M.E = emptyOn α
Tactic:
simp [← ground_eq_empty_iff]
State after:
no goals