assumption
case hstart
β : Type u_2
α : Type u_1
f : β → α → β
b : β
as : Array α
start stop : Nat
hstart : start < stop
hstop : stop ≤ as.size
⊢ start < stop

case hstop
β : Type u_2
α : Type u_1
f : β → α → β
b : β
as : Array α
start stop : Nat
hstart : start < stop
hstop : stop ≤ as.size
⊢ stop ≤ as.size
case hstop
β : Type u_2
α : Type u_1
f : β → α → β
b : β
as : Array α
start stop : Nat
hstart : start < stop
hstop : stop ≤ as.size
⊢ stop ≤ as.size