have hstart' : start < stop := Nat.lt_of_le_of_ne hstart hne
α : Type u_1
as bs : Array α
start stop : Nat
hstart : start ≤ stop
hstop : stop ≤ bs.size
hne : ¬start = stop
⊢ (foldl push as bs start stop).data = as.data ++ bs.data.extract start stop
α : Type u_1
as bs : Array α
start stop : Nat
hstart : start ≤ stop
hstop : stop ≤ bs.size
hne : ¬start = stop
hstart' : start < stop
⊢ (foldl push as bs start stop).data = as.data ++ bs.data.extract start stop