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