split at h
α : Type u_1
as : Array α
w✝ : α
h : (if h : 0 < size as then some as[0] else none) = some w✝
⊢ ¬size as = 0
case inl
α : Type u_1
as : Array α
w✝ : α
h✝ : 0 < size as
h : some as[0] = some w✝
⊢ ¬size as = 0

case inr
α : Type u_1
as : Array α
w✝ : α
h✝ : ¬0 < size as
h : False
⊢ ¬size as = 0