rw [relRank, ← er_cl_eq, contract_cl_eq, ← relRank_eq_er_diff_contract, relRank_cl_right]
α : Type u_1
M✝ N : Matroid α
I J C D X✝ Y✝ Z : Set α
e f : α
M : Matroid α
X Y : Set α
⊢ M.relRank X Y = M.relRank X (Y ∪ X)
no goals