lift k to ℕ using hk
α : Type u_1
M : Matroid α
E I B X C : Set α
k✝ k : ℕ∞
hk : k ≠ ⊤
⊢ (unifOn E k).Spanning X ↔ k ≤ X.encard ∧ X ⊆ E ∨ X = E
case intro
α : Type u_1
M : Matroid α
E I B X C : Set α
k✝ : ℕ∞
k : ℕ
⊢ (unifOn E ↑k).Spanning X ↔ ↑k ≤ X.encard ∧ X ⊆ E ∨ X = E