rintro rfl
case mp
α : Type u_1
M : Matroid α
E I B X C : Set α
k : ℕ∞
⊢ M = unifOn E 2 → M.E = E ∧ M.erk ≤ 2 ∧ M.Simple
case mp
α : Type u_1
E I B X C : Set α
k : ℕ∞
⊢ (unifOn E 2).E = E ∧ (unifOn E 2).erk ≤ 2 ∧ (unifOn E 2).Simple