left
case pos
l : List ℕ
head : ℕ
tail : List ℕ
hyp : head :: tail ≠ []
c : ¬tail = []
c' : head ≤ smallest tail ⋯
⊢ head ≤ smallest tail ⋯ ∨ Nat.min head (smallest tail ⋯) ∈ tail
case pos.h
l : List ℕ
head : ℕ
tail : List ℕ
hyp : head :: tail ≠ []
c : ¬tail = []
c' : head ≤ smallest tail ⋯
⊢ head ≤ smallest tail ⋯