rw [restrict_cl_eq', empty_inter, union_empty_iff] at hC
α : Type u_1
M N : Matroid α
e f g : α
I X P D C : Set α
hC : (M ↾ C).cl ∅ = ∅
h : M.cl C ⊆ C
⊢ M.cl ∅ = ∅
α : Type u_1
M N : Matroid α
e f g : α
I X P D C : Set α
hC : M.cl ∅ ∩ C = ∅ ∧ C \ M.E = ∅
h : M.cl C ⊆ C
⊢ M.cl ∅ = ∅